Rework libcxx strerror_r handling.

The set of #ifdefs used to handle the two incompatible variants of
strerror_r were not complete (they didn't handle newlib appropriately).

Rather than attempting to make the ifdefs more complex, make them
unnecessary by choosing which behavior to use dependent upon the
return type.

Reviewers: waltl

Differential Revision: https://reviews.llvm.org/D34294

llvm-svn: 308528