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/y2016/dashboard/BUILD b/y2016/dashboard/BUILD
index 9e8198a..0dc3ad4 100644
--- a/y2016/dashboard/BUILD
+++ b/y2016/dashboard/BUILD
@@ -7,6 +7,7 @@
         ["www_defaults/**/*"],
         exclude = ["www/**/*"],
     ),
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 aos_downloader_dir(
@@ -15,6 +16,7 @@
         "www/**/*",
     ]),
     dir = "www",
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
@@ -24,6 +26,7 @@
         "dashboard.cc",
         "dashboard.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = [
         ":gen_embedded",