Make cacosh (0 + iNaN) return NaN + i pi/2 (bug 22561, DR#471).

Make cacosh (0 + iNaN) return NaN + i pi/2 (bug 22561, DR#471).

As per C11 DR#471 (adjusted resolution accepted for C17), cacosh (0 +
iNaN) should return NaN +/- i pi/2, not NaN + iNaN.

This patch fixes the code accordingly.  The test has hardcoded the
result with positive sign of the imaginary part (with an associated
comment), since the unspecified sign for a result other than 0 or
infinity isn't currently supported by the test infrastructure.

Tested for x86_64.

	[BZ #22561]
	* math/s_cacosh_template.c (M_DECL_FUNC (__cacosh)): Use pi/2 for
	real part of result for argument 0 + i * NaN.
	* math/libm-test-cacosh.inc (cacosh_test_data): Update expected
	results for tests of 0 + i * NaN.