Fix exp2 spurious underflows (bug 16560).

Fix exp2 spurious underflows (bug 16560).

This patch fixes the remaining part of bug 16560, spurious underflows
from exp2 of arguments close to 0 (when the result is close to 1, so
should not underflow), by just using 1+x instead of a more complicated
calculation when the argument is sufficiently small.

Tested for x86_64, x86 and mips64.

	[BZ #16560]
	* math/e_exp2l.c [LDBL_MANT_DIG == 106] (LDBL_EPSILON): Undefine
	and redefine.
	(__ieee754_exp2l): Do not multiply small fractional parts by
	M_LN2l.
	* sysdeps/i386/fpu/e_exp2l.S (__ieee754_exp2l): Just add 1 to
	small argument.
	* sysdeps/ieee754/dbl-64/e_exp2.c (__ieee754_exp2): Likewise.
	* sysdeps/ieee754/flt-32/e_exp2f.c (__ieee754_exp2f): Likewise.
	* sysdeps/x86_64/fpu/e_exp2l.S (__ieee754_exp2l): Likewise.
	* math/auto-libm-test-in: Add more tests of exp2.
	* math/auto-libm-test-out: Regenerated.