[libc] add shared mulsf3 builtin (#205678)
Re-exposes LLVM-libc's `__mulsf3` as `shared::mulsf3` for reuse by
compiler-rt's builtins.
Stacked change - merge these first:
- #200094
- #205669
- #205670
- #205671
- #205672
- #205673
- #205674
- #205675
- #205676
- #205677
Part of #197824
