Fix exp10 spurious underflows (bug 16560).

Fix exp10 spurious underflows (bug 16560).

This patch fixes spurious underflows from exp10 for arguments near 0
(part of bug 16560; that bug also includes spurious underflows from
exp2, which are not fixed by this patch).  The problem is underflows
in the internal computation converting the exp10 argument to arguments
for exp (with extra precision), and the fix is simply to return 1
early for arguments near enough to 0 (just as arguments with large
enough magnitude have their own overflow / underflow logic at the
start of the function).

Tested x86_64 and x86 and ulps updated accordingly; also tested for
powerpc32 and mips64 to validate the ldbl-128ibm and ldbl-128 changes.

	[BZ #16560]
	* sysdeps/ieee754/dbl-64/e_exp10.c (__ieee754_exp10): Return 1 for
	arguments close to 0.
	* sysdeps/ieee754/ldbl-128/e_exp10l.c (__ieee754_exp10l):
	Likewise.
	* sysdeps/ieee754/ldbl-128ibm/e_exp10l.c (__ieee754_exp10l):
	Likewise.
	* math/auto-libm-test-in: Add more tests of exp10.
	* math/auto-libm-test-out: Regenerated.
	* sysdeps/x86_64/fpu/libm-test-ulps: Update.