Correct IBM long double frexpl.

Besides fixing the bugzilla, this also fixes corner-cases where the high
and low double differ greatly in magnitude, and handles a denormal
input without resorting to a fp rescale.

	[BZ #16740]
	[BZ #16619]
	* sysdeps/ieee754/ldbl-128ibm/s_frexpl.c (__frexpl): Rewrite.
	* math/libm-test.inc (frexp_test_data): Add tests.

Backport of aa5f0ff1 and
9860b045.