powerpc64le: blacklist broken GCC compilers (e.g GCC 7.5.0)

GCC 7.5.0 (PR94200) will refuse to compile if both -mabi=% and
-mlong-double-128 are passed on the command line.  Surprisingly,
it will work happily if the latter is not.  For the sake of
maintaining status quo, test for and blacklist such compilers.

Tested with a GCC 8.3.1 and GCC 7.5.0 compiler for ppc64le.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>