XFAIL libm-test.inc tests as needed for ibm128.

XFAIL libm-test.inc tests as needed for ibm128.

This patch arranges for various libm-test.inc tests to be XFAILed for
ibm128-libgcc in non-default rounding modes.  The tests are marked
with XFAIL_ROUNDING_IBM128_LIBGCC and gen-libm-test.pl is made to
transform that to XFAIL_IBM128_LIBGCC or 0 depending on the rounding
mode.

This should allow test-ldouble, test-ildouble and test-ldouble-finite
to pass with unmodified libgcc, given an ulps regeneration.  (The case
of patched libgcc was already clean up to ulps and possibly hypot
cases very close to the overflow threshold that may need more
XFAILing; patched libgcc, which should work with
TEST_COND_ibm128_libgcc defined to 0 to disable all these XFAILs, does
need slightly different ulps from unpatched.)  Note that soft-float
powerpc will still fail because of
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64811> resulting in
spurious "invalid" exceptions in the libgcc code (for hard float,
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58684> hides that bug).

Tested for powerpc.

	* math/libm-test.inc (XFAIL_IBM128_LIBGCC): New macro.
	(fdim_test_data): Use XFAIL_ROUNDING_IBM128_LIBGCC for some tests.
	(fma_test_data): Likewise.
	(hypot_test_data): Likewise.
	(log1p_test_data): Likewise.
	(modf_test_data): Likewise.
	(pow_test_data): Likewise.
	(remainder_test_data): Likewise.
	(remquo_test_data): Likewise.
	(scalb_test_data): Likewise.
	(scalbn_test_data): Likewise.
	(scalbln_test_data): Likewise.
	* math/gen-libm-test.pl (parse_args): Transform
	XFAIL_ROUNDING_IBM128_LIBGCC to XFAIL_IBM128_LIBGCC or 0 depending
	on the rounding mode.