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/frc971/zeroing/BUILD b/frc971/zeroing/BUILD
index df85290..cd90e8d 100644
--- a/frc971/zeroing/BUILD
+++ b/frc971/zeroing/BUILD
@@ -1,12 +1,11 @@
 package(default_visibility = ["//visibility:public"])
 
-load("//tools:environments.bzl", "mcu_cpus")
-
 cc_library(
     name = "averager",
     hdrs = [
         "averager.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "@com_github_google_glog//:glog",
         "@org_tuxfamily_eigen//:eigen",
@@ -18,6 +17,7 @@
     srcs = [
         "averager_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":averager",
         "//aos/testing:googletest",
@@ -33,6 +33,7 @@
     hdrs = [
         "imu_zeroer.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":averager",
         "//frc971/control_loops:control_loops_fbs",
@@ -48,6 +49,7 @@
     srcs = [
         "imu_zeroer_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":imu_zeroer",
         "//aos:flatbuffers",
@@ -59,22 +61,23 @@
 cc_library(
     name = "zeroing",
     srcs = [
+        "absolute_and_absolute_encoder.cc",
         "absolute_encoder.cc",
         "hall_effect_and_position.cc",
         "pot_and_absolute_encoder.cc",
         "pot_and_index.cc",
         "pulse_index.cc",
-        "absolute_and_absolute_encoder.cc",
     ],
     hdrs = [
+        "absolute_and_absolute_encoder.h",
         "absolute_encoder.h",
         "hall_effect_and_position.h",
         "pot_and_absolute_encoder.h",
         "pot_and_index.h",
         "pulse_index.h",
         "zeroing.h",
-        "absolute_and_absolute_encoder.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":wrap",
         "//aos/logging",
@@ -87,15 +90,16 @@
 cc_test(
     name = "zeroing_test",
     srcs = [
+        "absolute_and_absolute_encoder_test.cc",
         "absolute_encoder_test.cc",
         "hall_effect_and_position_test.cc",
         "pot_and_absolute_encoder_test.cc",
         "pot_and_index_test.cc",
         "pulse_index_test.cc",
         "relative_encoder_test.cc",
-        "absolute_and_absolute_encoder_test.cc",
         "zeroing_test.h",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":zeroing",
         "//aos/testing:googletest",
@@ -113,7 +117,6 @@
     hdrs = [
         "wrap.h",
     ],
-    compatible_with = mcu_cpus,
 )
 
 cc_test(
@@ -121,6 +124,7 @@
     srcs = [
         "wrap_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":wrap",
         "//aos/testing:googletest",
@@ -132,6 +136,7 @@
     srcs = [
         "unwrap_test.cc",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         ":wrap",
         "//aos/testing:googletest",