Fix x86_64 / x86 expm1l (-min_subnorm) result sign (bug 18569).

Fix x86_64 / x86 expm1l (-min_subnorm) result sign (bug 18569).

In the x86 / x86_64 implementations of expm1l, when expm1l's result
should underflow to 0 (argument minus the least subnormal, in some
rounding modes), it can be a zero of the wrong sign.  This patch fixes
this by returning the argument with underflow forced in that case
(this is a 1ulp error relative to the correctly rounded result of -0,
which is OK in terms of the documented accuracy goals, whereas a
result with the wrong sign never is).

Tested for x86_64 and x86.

	[BZ #18569]
	* sysdeps/i386/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]: Force
	underflow and return argument in case of subnormal argument.
	* sysdeps/x86_64/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]:
	Likewise.
	* math/auto-libm-test-in: Add more tests of expm1.
	* math/auto-libm-test-out: Regenerated.