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/tools/BUILD b/tools/BUILD
index 50c9141..3acd443 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -7,44 +7,47 @@
     name = "compiler_clang",
     flag_values = {
         "@bazel_tools//tools/cpp:compiler": "clang",
-    }
+    },
 )
 
 config_setting(
     name = "compiler_gcc",
     flag_values = {
         "@bazel_tools//tools/cpp:compiler": "gcc",
-    }
+    },
 )
 
+# TODO(phil): Get rid of these and just use the constraint values directly
+# everywhere.
 config_setting(
     name = "cpu_k8",
-    values = {"cpu": "k8"},
+    constraint_values = ["@platforms//cpu:x86_64"],
 )
 
 config_setting(
     name = "cpu_roborio",
-    values = {"cpu": "roborio"},
+    constraint_values = ["@//tools/platforms/hardware:roborio"],
 )
 
 config_setting(
     name = "cpu_cortex_m4f",
-    values = {"cpu": "cortex-m4f"},
+    constraint_values = ["@//tools/platforms/hardware:cortex_m4f"],
 )
 
-config_setting(
-    name = "cpu_cortex_m4f_k22",
-    values = {"cpu": "cortex-m4f-k22"},
-)
+# TODO(phil): Re-enable this with proper support.
+#config_setting(
+#    name = "cpu_cortex_m4f_k22",
+#    constraint_values = ["@platforms//cpu:?"],
+#)
 
 config_setting(
     name = "cpu_armhf",
-    values = {"cpu": "armhf-debian"},
+    constraint_values = ["@//tools/platforms/hardware:raspberry_pi"],
 )
 
 config_setting(
     name = "cpu_aarch64",
-    values = {"cpu": "aarch64"},
+    constraint_values = ["@platforms//cpu:arm64"],
 )
 
 config_setting(
@@ -61,33 +64,3 @@
     name = "has_ubsan",
     values = {"define": "have_ubsan=true"},
 )
-
-environment(name = "k8")
-
-environment(name = "armhf-debian")
-
-environment(name = "roborio")
-
-environment(name = "cortex-m4f")
-
-environment(name = "cortex-m4f-k22")
-
-environment(name = "aarch64")
-
-environment_group(
-    name = "cpus",
-    defaults = [
-        ":k8",
-        ":roborio",
-        ":aarch64",
-        ":armhf-debian",
-    ],
-    environments = [
-        ":k8",
-        ":roborio",
-        ":armhf-debian",
-        ":cortex-m4f",
-        ":cortex-m4f-k22",
-        ":aarch64",
-    ],
-)