[libc] add shared subtf3 builtin (#205669)
Re-exposes LLVM-libc's `__subtf3` as `shared::subtf3` for reuse by
compiler-rt's builtins.
Stacked change - merge these first:
- #200094
Part of #197824
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Co-authored-by: Muhammad Bassiouni <60100307+bassiounix@users.noreply.github.com>
