compiler-rt: Fix signedness mismatch in f128 mul impl
The `1 - shift` expression was computed using small unsigned types and
then casted to i32, producing either an underflow error or an incorrect
result.
Reported by `@notviri` in #8733
5185b5619aLemonBoy committed on 5/14/2021, 7:11:54 PM· committed by Andrew Kelleyparent00ebbe6