apple_common.multi_arch_split should transition on platforms instead of crosstool_top

[steeve]

Description of the problem / feature request:

Let apple_common.multi_arch_split transition on platforms/constraints instead of crosstool_top/cpu.

Feature requests: what underlying problem are you trying to solve with this feature?

According to osx_archs.bzl, Apple toolchains are already constraint aware and thus can be selected by specifying a platform.

However, according to AppleCrosstoolTransition, apple_common.multi_arch_split is still implemented by transiting crosstool_top and cpu instead of platforms.

This creates weird cases for downstream rules which have to transition on crosstool_top/cpu instead of letting the constraints apply directly to their toolchains as well.
For instance, rules could expose CcInfo and thus be used as dependencies of a rule using multi_arch_split.
See bazel-contrib/rules_go#2451.

Looking at rules_apple, I see the starlark transitions are are implemented but apple_common.multi_arch_split is still used.

What operating system are you running Bazel on?

macOS

What's the output of bazel info release?

release 3.0.0

Any other information, logs, or outputs that you want to share?

1. Add automatic platform detection from inbound crosstool_top and cpu bazel-contrib/rules_go#2451

[jmmv]

@katre , could you triage? I see you are involved in the review of #2451 and I know little about this stuff right now.

[steeve]

Closing, wrong dup of #11180. Sorry for the noise.