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/protobuf/protobuf.bzl b/third_party/protobuf/protobuf.bzl
index a125606..ae3ea54 100644
--- a/third_party/protobuf/protobuf.bzl
+++ b/third_party/protobuf/protobuf.bzl
@@ -209,7 +209,7 @@
         internal_bootstrap_hack=False,
         use_grpc_plugin=False,
         default_runtime="@com_google_protobuf//:protobuf",
-        compatible_with = None,
+        target_compatible_with = None,
         copts = [],
         **kargs):
   """Bazel rule to create a C++ protobuf library from proto source files
@@ -277,7 +277,7 @@
       plugin_language="grpc",
       gen_cc=1,
       outs=outs,
-      compatible_with = compatible_with,
+      target_compatible_with = target_compatible_with,
       visibility=["//visibility:public"],
   )
 
@@ -293,7 +293,7 @@
       deps=cc_libs + deps,
       includes=includes,
       copts = COPTS + copts,
-      compatible_with = compatible_with,
+      target_compatible_with = target_compatible_with,
       **kargs)
 
 def internal_gen_well_known_protos_java(srcs):