powerpc64le: Ensure correct ldouble compiler flags are used

Ensure the correct ldouble abi flags are applied to ibm128 files and
nldbl files.  Remove the IEEE options if used, and apply the flags
used to build ldouble files which are ibm128 abi.

nldbl tests are a little tricky.  To use the support, we must remove
all ldouble abi flags, and ensure -mlong-double-64 is used.

Co-authored-by: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Co-authored-by: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
Co-authored-by: Paul E. Murphy <murphyp@linux.vnet.ibm.com>