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/aos/time/BUILD b/aos/time/BUILD
index c9f8d6d..c0bd99d 100644
--- a/aos/time/BUILD
+++ b/aos/time/BUILD
@@ -1,5 +1,3 @@
-load("//tools:environments.bzl", "mcu_cpus")
-
 cc_library(
     name = "time",
     srcs = [
@@ -9,25 +7,10 @@
         "time.h",
     ],
     visibility = ["//visibility:public"],
-    deps = [
-        "@com_github_google_glog//:glog",
-    ],
-)
-
-# TODO(Brian): Remove this hack once bazel chases deps through selects correctly.
-cc_library(
-    name = "time_mcu",
-    srcs = [
-        "time.cc",
-    ],
-    hdrs = [
-        "time.h",
-    ],
-    restricted_to = mcu_cpus,
-    visibility = ["//visibility:public"],
-    deps = [
-        "//motors/core",
-    ],
+    deps = select({
+        "@platforms//os:linux": ["@com_github_google_glog//:glog"],
+        "//conditions:default": ["//motors/core"],
+    }),
 )
 
 cc_test(
@@ -35,6 +18,7 @@
     srcs = [
         "time_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":time",
         "//aos/logging",