Fix building DynamicLibrary.cpp with musl libc

Summary:
The workaround added in rL301240 for stderr/out/in symbols being both
macros and globals is only necessary for glibc, and it does not compile
with musl libc. Alpine Linux has had the following fix for it:

https://git.alpinelinux.org/cgit/aports/plain/main/llvm4/llvm-fix-DynamicLibrary-to-build-with-musl-libc.patch

Adapt the fix in our DynamicLibrary.inc for Unix.

Reviewers: marsupial, chandlerc, krytarowski

Reviewed By: krytarowski

Subscribers: srhines, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D33883

llvm-svn: 304707