[libc] add shared subsf3 builtin (#205677)

Re-exposes LLVM-libc's `__subsf3` as `shared::subsf3` for reuse by
compiler-rt's builtins.

Stacked change - merge these first:
- #200094
- #205669
- #205670
- #205671
- #205672
- #205673
- #205674
- #205675
- #205676

Part of #197824

NOKEYCHECK=True
GitOrigin-RevId: 0ad19f403165d8fc1596fb2fef9c940e941c54df