Auto merge of #253 - nodakai:strerror_r, r=alexcrichton

On non-musl Linux, strerror_r should be linked to __xpg_strerror_r

Currently `libc::strerror_r()` wrongly returns a `c_char` pointer as a large `c_int`.

Also exclude `MS_RMT_MASK` from `libc-test`.