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/frc971/BUILD b/frc971/BUILD
index 563f05e..d6a3af8 100644
--- a/frc971/BUILD
+++ b/frc971/BUILD
@@ -1,13 +1,10 @@
 package(default_visibility = ["//visibility:public"])
 
-load("//tools:environments.bzl", "mcu_cpus")
-
 cc_library(
     name = "shifter_hall_effect",
     hdrs = [
         "shifter_hall_effect.h",
     ],
-    compatible_with = mcu_cpus,
 )
 
 cc_library(
@@ -15,10 +12,12 @@
     hdrs = [
         "constants.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 py_library(
     name = "python_init",
     srcs = ["__init__.py"],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )