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/flatbuffers/BUILD b/third_party/flatbuffers/BUILD
index 3114495..57f2b8e 100644
--- a/third_party/flatbuffers/BUILD
+++ b/third_party/flatbuffers/BUILD
@@ -1,11 +1,9 @@
-licenses(["notice"])
-
-load("@//tools:environments.bzl", "mcu_cpus")
-
 package(
     default_visibility = ["//visibility:public"],
 )
 
+licenses(["notice"])
+
 exports_files([
     "LICENSE",
     "tsconfig.json",
@@ -79,7 +77,6 @@
         "include/flatbuffers/stl_emulation.h",
         "include/flatbuffers/util.h",
     ],
-    compatible_with = mcu_cpus,
     linkstatic = 1,
     strip_include_prefix = "/include",
 )