bazel/platforms: Setups and cleanups

[phlax]

this does some groundwork for switching to hermetic llvm toolchains

• move platform registration to bazel/platforms
• add BAZEL_LLVM_PATH var that can be set via repo_env for setting local llvm install
• add @mobile_clang_platform

[repokitteh-read-only]

CC @envoyproxy/dependency-shepherds: Your approval is needed for changes made to (bazel/.*repos.*\.bzl)|(bazel/dependency_imports\.bzl)|(api/bazel/.*\.bzl)|(.*/requirements\.txt)|(.*\.patch).
envoyproxy/dependency-shepherds assignee is @RyanTheOptimist

🐱

Caused by: #42202 was opened by phlax.

see: more, trace.

[phlax]

cc @jwendell @zirain @abeyad @danzh2010

this adds a repo env var BAZEL_LLVM_PATH - its not used yet - but when its hooked up this will be the way to use a locally installed llvm - ie

bazel build --repo_env=BAZEL_LLVM_PATH=/path/to/llvm --config=clang //some:target

i expect using this to be a rare exception tho - because the default (ie --confg=clang) will just pull in the correct toolchain so the need to install locally should be mostly gone

[phlax]

/retest