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/third_party/flatbuffers/tests/BUILD b/third_party/flatbuffers/tests/BUILD
index 203a08c..6fbda6b 100644
--- a/third_party/flatbuffers/tests/BUILD
+++ b/third_party/flatbuffers/tests/BUILD
@@ -14,13 +14,13 @@
         "namespace_test/namespace_test2_generated.h",
         "native_type_test_impl.cpp",
         "native_type_test_impl.h",
+        "optional_scalars_generated.h",
         "test.cpp",
         "test_assert.cpp",
         "test_assert.h",
         "test_builder.cpp",
         "test_builder.h",
         "union_vector/union_vector_generated.h",
-        "optional_scalars_generated.h",
     ],
     copts = [
         "-DFLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE",
@@ -43,6 +43,7 @@
         ":monsterdata_test.golden",
         ":monsterdata_test.json",
         ":native_type_test.fbs",
+        ":optional_scalars.fbs",
         ":prototest/imported.proto",
         ":prototest/test.golden",
         ":prototest/test.proto",
@@ -54,7 +55,6 @@
         ":unicode_test.json",
         ":union_vector/union_vector.fbs",
         ":union_vector/union_vector.json",
-        ":optional_scalars.fbs",
     ],
     includes = [
         "",