powerpc64: apply -mabi=ibmlongdouble to special files

Some of these files depend on the avoidance of using the various
register sets of POWER.  When enabling the IEEE 128 long double,
we must be sure to disable this ABI as some compilers will
refuse to compile if -mno-vsx and -mabi=ieeelongdouble are both
present.

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