Revert "Define _GNU_SOURCE for rtems c++"

This reverts commit r305399.

This breaks a build in libcxx:
libcxx/src/system_error.cpp:90:16: error: assigning to 'int' from incompatible type 'char *'
    if ((ret = ::strerror_r(ev, buffer, strerror_buff_size)) != 0) {
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Which makes sense according to:
https://linux.die.net/man/3/strerror_r

Not entirely sure how this needs to be fixed.

llvm-svn: 305456