compiler-rt: allow golang race detector to run on musl-c

tsan while used by golang's race detector was not working on alpine
linux, since it is using musl-c instead of glibc. Since alpine is very
popular distribution for container deployments, having working race
detector would be nice. This commits adds some ifdefs to get it working.

It fixes https://github.com/golang/go/issues/14481 on golang's issue tracker.

Reviewed-in: https://reviews.llvm.org/D75849
Author: graywolf-at-work (Tomas Volf)