Use correct function signature for strerror_r interceptor

There are two possible return values for strerror_r:

On OS X, the return value is always `int`.
On Linux, the return value can be either `char *` or `int`, depending
on the value of:
`(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE`

Because OS X interceptors require a matching function signature,
split out the two cases into separate interceptors, using the above
information to determine the correct signature for a given build.

llvm-svn: 297315