overhaul musl support

[kaniini]

As discussed in Zulip (https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/bootstrapping.20on.20s390x.2Fmips.20for.20musl/near/227979341), there are numerous opportunities to simplify the musl support crate (almost everything is actually not arch-specific in musl, so there is a ton of DRY violations).

@workingjubilee observed that we should layer things like musl does, e.g. everything that is common goes in the common part, and then everything that is in the arch/$arch/bits headers goes in arch-specific crates.

I think we should also take this opportunity to finish the time64 migration for 32-bit hosts and assume musl 1.2.x as baseline.

[JohnTitor]

Could you provide the Zulip stream link?

[jyn514]

Could you provide the Zulip stream link?

https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/bootstrapping.20on.20s390x.2Fmips.20for.20musl/near/227979341