add mulXf3 to compiler-rt

this adds the following functions to compiler-rt:

 * `__mulsf3`
 * `__muldf3`
 * `__multf3`

See #1290
d83836825fAndrew Kelley committed on 3/22/2019, 9:46:49 PMparent324cbb9
5 files changedLine totals unavailable