Generate native Bazel platforms instead of requiring tint_build_is flags

Currently, people who use the Bazel rules for tint and dawn need to
specify os flags like --//src/tint:os=win when running *every*
bazel command. This is not very idiomatic (unlike GN where build flags
are the standard) and Skia has found this cumbersome.

This change replaces those hand-specified flags with the built-in
"platforms" attributes. These automatically get set based on the
target OS (which defaults to the host os). These will be easier
for clients (such as Skia) to use.

This also cleans up bazel labels - for labels defined in the same
file (such as the tint_build_wgsl_writer aliases) they are
prefixed with a colon which is better stylistically.

Turns out the mac rules were broken - you can't put a .mm file
in a cc_library. I've temporarily removed those and plan to
have a follow-on CL that creates objc_library targets that work
with Bazel.

Suggested review order:
 - src/tint/utils/command/BUILD.bazel to see examples where conditions
   involving one platform (windows) are a direct swap and also cases
   where it's a "platforms *except* for these" are handled with a
   more idiomatic structure.
 - All the rest of the BUILD.bazel files to see the changes are
   equivalent
 - build.go and expr.go (probably start with the tests) to see
   the new helper functions
 - tools/src/cmd/gen/build/BUILD.bazel.tmpl which uses the new
   helper functions
 - Anything else

Change-Id: I61ae06a3f56f0f4ade65c43d6813a3448e0ccdcd
Bug: b/256859233
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/324475
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>