Fix a compile error on musl-libc due to strerror_r() prototype

Function strerror_r() has different signatures in different
implementations of libc: glibc's version returns a char*, while BSDs
and musl return a int. libomp unconditionally assumes glibc on Linux
and thus fails to compile against musl-libc. This patch addresses this
issue.

Differential Revision: https://reviews.llvm.org/D25071

llvm-svn: 284492