Remove NO_LONG_DOUBLE conditionals in libm tests (bug 21607).

As noted in bug 21607, NO_LONG_DOUBLE conditionals in libm tests are
no longer effective.  For most this is harmless - they were only
present because of long double functions not being declared with _LIBC
defined, and _LIBC is no longer defined for building most tests.  For
the few where this is actually relevant to the test, testing
LDBL_MANT_DIG > DBL_MANT_DIG is more appropriate as that limits the
test to public APIs.  This patch fixes the tests accordingly.

Tested for x86_64 and arm.

	[BZ #21607]
	* math/basic-test.c [!NO_LONG_DOUBLE]: Change conditionals to
	[LDBL_MANT_DIG > DBL_MANT_DIG].
	* math/bug-nextafter.c [!NO_LONG_DOUBLE]: Remove conditionals.
	* math/bug-nexttoward.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-math-isinff.cc [!NO_LONG_DOUBLE]: Likewise.
	* math/test-math-iszero.cc [!NO_LONG_DOUBLE]: Likewise.
	* math/test-nan-overflow.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-nan-payload.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-nearbyint-except-2.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-nearbyint-except.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-powl.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-signgam-finite-c99.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-signgam-finite.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-signgam-main.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-snan.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-tgmath-ret.c [!NO_LONG_DOUBLE]: Likewise.
	* math/test-tgmath.c: Include <float.h>.
	[!NO_LONG_DOUBLE]: Change conditionals to [LDBL_MANT_DIG >
	DBL_MANT_DIG].
	* math/test-tgmath2.c: Include <float.h>.
	[!NO_LONG_DOUBLE]: Change conditionals to [LDBL_MANT_DIG >
	DBL_MANT_DIG].