Add Makefile infrastructure and IBM 128-bit 'long double' real input for
targets switching between the IEEE 754 binary128 and IBM 128-bit formats
with '-mabi=ieeelongdouble' and '-mabi=ibmlongdouble'. Reuse IEEE 754
binary128 input data but with modified output file names so as not to
clash with the names used for IBM 128-bit format tests made with common
rules for the 'long double' data type.
Keep input data disabled and referring to BZ #12701 for entries that are
are currently incorrectly accepted as valid data, such as '0e', '0e+',
'0x', '0x8p', '0x0p-', etc.
Reviewed-by: Joseph Myers <josmyers@redhat.com>
