Set target_cpu in toolchain

[keith]

This is currently usable in select statements (which is deprecated but
won't be flipped for 1 year) and also is used internally by bazel for
some paths like _solib_k8. We can derive this from platform configs

[armandomontanez]

I'd really prefer we don't carry this one forward, and instead fix any instances that rely on target_cpu to ensure they're controlled through a more future-looking mechanism.

[keith]

I agree with that in theory. But given bazel's long deprecation timelines it seems to me that the cost of us carrying it along for a while with a change like this, is very much worth it compared to the migration cost to make sure your transitive closure avoids needing this.

In our moderately sized C++ project at least grpc: grpc/grpc#38319

protobuf: https://github.com/protocolbuffers/protobuf/blob/71c259485c2a40105144c4f0f0d1910a6ae0d169/build_defs/BUILD.bazel#L35-L38

and abseil: https://github.com/abseil/abseil-cpp/pull/1797/files

all use this mechanism. So my thinking is that even if we snapped our fingers and fixed all of today's deprecated uses, it's non-trivial to upgrade these dependencies in any projects that potentially want to use this API.

I'm also quite worried about moving bazel off of this API, here are a few places I found that have been using it forever:

• https://github.com/bazelbuild/bazel/blob/97e125f0489379e4b402edaee873f974ae7a7dd0/src/main/starlark/builtins_bzl/common/cc/cc_toolchain_provider_helper.bzl#L197
• https://github.com/bazelbuild/bazel/blob/97e125f0489379e4b402edaee873f974ae7a7dd0/src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationValue.java#L302-L313
• https://github.com/bazelbuild/bazel/blob/97e125f0489379e4b402edaee873f974ae7a7dd0/src/main/starlark/builtins_bzl/common/cc/fdo/fdo_context.bzl#L267-L272