Fix x86_64/x86 powl handling of sNaN arguments (bug 20916).

The x86_64/x86 powl implementations mishandle sNaN arguments, both by
returning sNaN in some cases (instead of doing arithmetic on the
arguments to produce the result when NaN arguments result in NaN
results) and by treating sNaN the same as qNaN for arguments (1, sNaN)
and (sNaN, 0), contrary to TS 18661-1 which requires those cases to
return qNaN instead of 1.

This patch makes the x86_64/x86 powl implementations follow TS 18661-1
semantics for sNaN arguments; sNaN tests are also added for pow.
Given the problems with testing float and double sNaN arguments on
32-bit x86 (sNaN tests disabled because the compiler may convert
unnecessarily to a qNaN when passing arguments), no changes are made
to the powf and pow implementations there.

Tested for x86_64 and x86.

	[BZ #20916]
	* sysdeps/i386/fpu/e_powl.S (__ieee754_powl): Do not return 1 for
	arguments (sNaN, 0) or (1, sNaN).  Do arithmetic on NaN arguments
	to compute result.
	* sysdeps/x86_64/fpu/e_powl.S (__ieee754_powl): Likewise.
	* math/libm-test.inc (pow_test_data): Add tests of sNaN arguments.