Relax gen-auto-libm-tests may-underflow rules, test log1p in all rounding modes.

Relax gen-auto-libm-tests may-underflow rules, test log1p in all rounding modes.

gen-auto-libm-tests presently allows but does not require underflow
exceptions for results with magnitude in the range (greatest
subnormal, least normal].

In some cases, the magnitude of the exact result is very slightly
above the least normal, but rounding in the implementation results in
it effectively computing an infinite-precision result that is slightly
below the least normal, so raising an underflow exception.  This is in
accordance with the documented accuracy goals, but results in
testsuite failures.

This patch changes the logic to allow underflows when the mathematical
result is up to 0.5ulp above the least normal (so in any case where
the round-to-nearest result is the least normal).  Ideally underflows
in all these cases would be accepted only when an underflow with the
actual result is consistent with the rounding mode (in FE_TOWARDZERO
mode, a return value of the least normal implies that the
infinite-precision result did not underflow so there should be no
underflow exception, for example), so as to match the documented goals
more precisely - whereas at present the tests for exceptions are
completely independent of the tests of the returned values.  (The same
applies to overflow exceptions as well - they too should be checked
for consistency with the result, as in FE_TOWARDZERO mode a result
1ulp below the largest finite value should be inconsistent with an
overflow exception and cause a failure with overflow rather than
simply being considered a 1ulp error when overflow is expected.)  But
the present patch at least deals with the cases causing spurious
failures so that (a) certain existing tests no longer need to be
marked as having spurious exceptions (such markings in
auto-libm-test-in end up applying to more cases than just those they
are needed for) and (b) log1p can be tested in all rounding modes
without introducing more such failures.  This patch duly moves tests
of log1p to ALL_RM_TEST.

Tested x86_64 and x86 and ulps updated accordingly.

	[BZ #16357]
	[BZ #16599]
	* math/gen-auto-libm-tests.c (fp_format_desc): Add field
	min_plus_half.
	(fp_formats): Update initializers.
	(init_fp_formats): Initialize new field.
	(output_for_one_input_case): Allow underflow for results up to
	min_plus_half.
	* math/libm-test.inc (log1p_test): Use ALL_RM_TEST.
	* math/auto-libm-test-in: Don't mark some underflows from asin and
	atanh as spurious.
	* math/auto-libm-test-out: Regenerated.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.