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/y2020/vision/sift/fast_gaussian.bzl b/y2020/vision/sift/fast_gaussian.bzl
index 0905423..5eaa4bf 100644
--- a/y2020/vision/sift/fast_gaussian.bzl
+++ b/y2020/vision/sift/fast_gaussian.bzl
@@ -1,3 +1,6 @@
+load("//tools:platforms.bzl", "platforms")
+load("//tools/build_rules:select.bzl", "cpu_select")
+
 def fast_gaussian(sigmas, sizes):
     files = []
     for _, sigma_name, _ in sigmas:
@@ -28,17 +31,22 @@
             ":fast_gaussian_runner",
         ],
         cmd = " ".join([
-            "$(location fast_gaussian_runner)",
+            "$(location :fast_gaussian_runner)",
             "'" + params.to_json() + "'",
-            # TODO(Brian): This should be RULEDIR once we have support for that.
-            "$(@D)",
-            "$(TARGET_CPU)",
-        ]),
+            "$(RULEDIR)",
+        ]) + " " + cpu_select({
+            "amd64": "k8",
+            "roborio": "roborio",
+            "armhf": "armhf-debian",
+            "cortex-m": "cortex-m",
+        }),
         outs = headers + objects + htmls,
-        restricted_to = [
-            "//tools:k8",
-            "//tools:armhf-debian",
-        ],
+        # The tool doesn't support anything other than k8 and armhf-debian
+        # right now.
+        target_compatible_with = platforms.any_of([
+            "@platforms//cpu:x86_64",
+            "//tools/platforms/hardware:raspberry_pi",
+        ]),
     )
 
     native.cc_library(
@@ -48,8 +56,4 @@
         deps = [
             "//third_party:halide_runtime",
         ],
-        restricted_to = [
-            "//tools:k8",
-            "//tools:armhf-debian",
-        ],
     )