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/seasocks/gen_embedded.bzl b/aos/seasocks/gen_embedded.bzl
index bb27e52..7d67249 100644
--- a/aos/seasocks/gen_embedded.bzl
+++ b/aos/seasocks/gen_embedded.bzl
@@ -41,11 +41,12 @@
   srcs: Files to allow loading.
 """
 
-def gen_embedded(name, srcs, visibility = None):
+def gen_embedded(name, srcs, visibility = None, target_compatible_with = None):
     _do_gen_embedded(
         name = name + "__do_gen",
         visibility = ["//visibility:private"],
         srcs = srcs,
+        target_compatible_with = target_compatible_with,
     )
     native.cc_library(
         name = name,
@@ -57,4 +58,5 @@
         deps = [
             "@//third_party/seasocks",
         ],
+        target_compatible_with = target_compatible_with,
     )