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/motors/macros.bzl b/motors/macros.bzl
index 6b15f00..0ff7c8d 100644
--- a/motors/macros.bzl
+++ b/motors/macros.bzl
@@ -1,4 +1,4 @@
-def hex_from_elf(name, restricted_to = None):
+def hex_from_elf(name, target_compatible_with = None):
     native.genrule(
         name = name,
         srcs = ["%s.elf" % name],
@@ -6,6 +6,6 @@
         cmd = "$(OBJCOPY) -O ihex $< $@",
         executable = True,
         output_to_bindir = True,
-        restricted_to = restricted_to,
+        target_compatible_with = target_compatible_with,
         toolchains = ["@bazel_tools//tools/cpp:current_cc_toolchain"],
     )