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/print/BUILD b/motors/print/BUILD
index 26d8e45..df27ce2 100644
--- a/motors/print/BUILD
+++ b/motors/print/BUILD
@@ -1,11 +1,9 @@
-load("//tools:environments.bzl", "mcu_cpus")
-
 cc_library(
     name = "print",
     hdrs = [
         "print.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         "//aos/containers:sized_array",
@@ -22,7 +20,7 @@
     hdrs = [
         "uart.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":print",
@@ -39,7 +37,7 @@
     hdrs = [
         "itm.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":print",
@@ -55,7 +53,7 @@
     hdrs = [
         "semihosting.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":print",
@@ -71,7 +69,7 @@
     hdrs = [
         "usb.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":print",