Fix ldbl-128ibm acoshl inaccuracy (bug 18019).

The ldbl-128ibm implementation of acoshl uses a cut-off of 0x1p28 to
determine when to use log(x) + log(2) as a formula.  That cut-off is
too small for this format, resulting in large errors.  This patch
changes it to a more appropriate cut-off of 0x1p56, adding tests
around the cut-offs for various floating-point formats.

Tested for powerpc.  Also tested for x86_64 and x86 and updated ulps.

	[BZ #18019]
	* sysdeps/ieee754/ldbl-128ibm/e_acoshl.c (__ieee754_acoshl): Use
	2**56 not 2**28 as threshold for log (2x) formula.
	* math/auto-libm-test-in: Add more tests of acosh.
	* math/auto-libm-test-out: Regenerated.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.