Switch everything to platforms

This patch switches the codebase over from using the "cpu"
mechanism to using bazel platforms. See
https://docs.bazel.build/versions/master/platforms.html for some more
information.

Most of the substantial changes are in //tools. Instead of using
`cc_toolchain_suite` rules, we now use regular `toolchain` rules that
are registered in the WORKSPACE. That also means that bazel now uses
the target platform to select the compiler.

All --cpu=* arguments should now be --config=* arguments. For example,
`--cpu=roborio` should now be `--config=roborio`. The CI script and
all documentation has been updated to reflect that.

The remainder of the changes revolve around tagging all targets with
`target_compatible_with`.  The old mechanism allowed us to specify
repo-wide defaults. The new mechanism does not. That means every
target that didn't have any compatibility specified, now requires
compatibility with `@platforms//os:linux`.

I used buildozer for the vast majority of `target_compatible_with`
changes. buildozer automatically buildifies any BUILD files it
touches. That means this patch also contains a few non-functional
changes that I was too lazy to remove.

Change-Id: I66d6e6ad9161520ee397597cdb492585820a3acd
diff --git a/third_party/BUILD b/third_party/BUILD
index 8bd83d7..4ecad0d 100644
--- a/third_party/BUILD
+++ b/third_party/BUILD
@@ -1,23 +1,30 @@
+load("//tools:platforms.bzl", "platforms")
 load("//tools/build_rules:select.bzl", "cpu_select")
 
+# Incompatible library in case one isn't available for a specific architecture.
+cc_library(
+    name = "unavailable",
+    target_compatible_with = ["@platforms//:incompatible"],
+)
+
 cc_library(
     name = "wpilib",
     linkstatic = True,
-    restricted_to = ["//tools:roborio"],
+    target_compatible_with = ["//tools/platforms/hardware:roborio"],
     visibility = ["//visibility:public"],
     deps = ["//frc971/wpilib/ahal"],
 )
 
 cc_library(
     name = "wpilib_hal",
-    restricted_to = ["//tools:roborio"],
+    target_compatible_with = ["//tools/platforms/hardware:roborio"],
     visibility = ["//visibility:public"],
     deps = ["//third_party/allwpilib:hal"],
 )
 
 cc_library(
     name = "phoenix",
-    restricted_to = ["//tools:roborio"],
+    target_compatible_with = ["//tools/platforms/hardware:roborio"],
     visibility = ["//visibility:public"],
     deps = [
         "@ctre_phoenix_api_cpp_athena//:api-cpp",
@@ -31,80 +38,61 @@
 
 cc_library(
     name = "opencv",
-    restricted_to = [
-        "//tools:k8",
-        "//tools:armhf-debian",
-    ],
     visibility = ["//visibility:public"],
     deps = select({
         "//tools:cpu_k8": ["@opencv_k8//:opencv"],
         "//tools:cpu_armhf": ["@opencv_armhf//:opencv"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )
 
 cc_library(
     name = "gstreamer",
-    restricted_to = [
-        "//tools:k8",
-        "//tools:armhf-debian",
-    ],
     visibility = ["//visibility:public"],
     deps = select({
         "//tools:cpu_k8": ["@gstreamer_k8//:gstreamer"],
         "//tools:cpu_armhf": ["@gstreamer_armhf//:gstreamer"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )
 
 cc_library(
     name = "halide",
-    restricted_to = [
-        "//tools:k8",
-        "//tools:armhf-debian",
-    ],
     visibility = ["//visibility:public"],
     deps = select({
         "//tools:cpu_k8": ["@halide_k8//:halide"],
         "//tools:cpu_armhf": ["@halide_armhf//:halide"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )
 
 cc_library(
     name = "halide_gengen",
-    restricted_to = [
-        "//tools:k8",
-        "//tools:armhf-debian",
-    ],
     visibility = ["//visibility:public"],
     # It's the same file in either version, but we'll pick the native version
     # to minimize the chances of needing to download the other version unnecessarily.
     deps = select({
         "//tools:cpu_k8": ["@halide_k8//:gengen"],
         "//tools:cpu_armhf": ["@halide_armhf//:gengen"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )
 
 cc_library(
     name = "halide_runtime",
-    restricted_to = [
-        "//tools:k8",
-        "//tools:armhf-debian",
-    ],
     visibility = ["//visibility:public"],
     # It's the same file in either version, but we'll pick the native version
     # to minimize the chances of needing to download the other version unnecessarily.
     deps = select({
         "//tools:cpu_k8": ["@halide_k8//:runtime"],
         "//tools:cpu_armhf": ["@halide_armhf//:runtime"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )
 
 cc_library(
     name = "webrtc",
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = cpu_select({
         "amd64": ["@webrtc_x64//:webrtc"],
@@ -114,14 +102,12 @@
     }),
 )
 
-# TODO(Brian): Properly restrict this to specific platforms and remove the
-# default deps once we have support for that which interacts with select properly.
 cc_library(
     name = "lzma",
     visibility = ["//visibility:public"],
     deps = select({
         "//tools:cpu_k8": ["@lzma_amd64//:lib"],
         "//tools:cpu_aarch64": ["@lzma_arm64//:lib"],
-        "//conditions:default": [],
+        "//conditions:default": [":unavailable"],
     }),
 )