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/usb/BUILD b/motors/usb/BUILD
index 3e92a16..1446e91 100644
--- a/motors/usb/BUILD
+++ b/motors/usb/BUILD
@@ -1,5 +1,3 @@
-load("//tools:environments.bzl", "mcu_cpus")
-
 cc_library(
     name = "legacy",
     srcs = [
@@ -18,7 +16,7 @@
     defines = [
         "USB_SERIAL=1",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         "//motors/core",
@@ -33,7 +31,7 @@
     hdrs = [
         "usb.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":constants",
@@ -51,7 +49,7 @@
     hdrs = [
         "cdc.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":queue",
@@ -69,7 +67,7 @@
     hdrs = [
         "interrupt_out.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":usb",
@@ -86,7 +84,6 @@
     hdrs = [
         "queue.h",
     ],
-    compatible_with = mcu_cpus,
 )
 
 cc_test(
@@ -94,6 +91,7 @@
     srcs = [
         "queue_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":queue",
         "//aos/testing:googletest",
@@ -105,7 +103,6 @@
     hdrs = [
         "constants.h",
     ],
-    compatible_with = mcu_cpus,
 )
 
 cc_test(
@@ -113,6 +110,7 @@
     srcs = [
         "constants_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":constants",
         "//aos/testing:googletest",
@@ -127,7 +125,7 @@
     hdrs = [
         "hid.h",
     ],
-    restricted_to = mcu_cpus,
+    target_compatible_with = ["@platforms//os:none"],
     visibility = ["//visibility:public"],
     deps = [
         ":usb",