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/debian/packages.bzl b/debian/packages.bzl
index 987f77f..2914388 100644
--- a/debian/packages.bzl
+++ b/debian/packages.bzl
@@ -24,7 +24,7 @@
 
 # TODO(phil): Deal with armhf packages. Right now only works for amd64.
 
-def download_packages(name, packages, excludes = [], force_includes = []):
+def download_packages(name, packages, excludes = [], force_includes = [], target_compatible_with = None):
     """Downloads a set of packages as well as their dependencies.
 
     You can also specify excludes in case some of the dependencies are meta
@@ -52,6 +52,7 @@
         # would like it to.
         cmd = "$(location //debian:download_packages) %s %s %s | tee $@ >&2" %
               (force_includes, excludes_list, package_list),
+        target_compatible_with = target_compatible_with,
     )
 
 def _convert_deb_to_target(deb):
@@ -80,7 +81,7 @@
                 downloaded_file_path = f,
             )
 
-def generate_deb_tarball(name, files):
+def generate_deb_tarball(name, files, target_compatible_with = None):
     """Takes all debs in the dictionary and generates one tarball from them.
 
     This can then be uploaded and used as another WORKSPACE entry.
@@ -95,10 +96,12 @@
                 srcs = ["@%s//file" % dep],
                 outs = ["extracted_%s.tar" % dep],
                 cmd = "dpkg-deb --fsys-tarfile $(SRCS) > $@",
+                target_compatible_with = target_compatible_with,
             )
 
     pkg_tar(
         name = name,
         extension = "tar.gz",
         deps = ["extracted_%s.tar" % dep for dep in deps],
+        target_compatible_with = target_compatible_with,
     )