Fix nextafter overflow in non-default rounding modes (bug 16677).

ISO C requires the result of nextafter to be independent of the
rounding mode, even when underflow or overflow occurs.  This patch
fixes the bug in various nextafter implementations that, having done
an overflowing computation to force an overflow exception (correct),
they then return the result of that computation rather than an
infinity computed some other way (incorrect, when the overflowing
result of arithmetic with that sign and rounding mode is finite but
the correct result is infinite) - generally by falling through to
existing code to return a value that in fact is correct for this case
(but was computed by an integer increment and so without generating
the exceptions required).  Having fixed the bug, the previously
deferred conversion of nextafter testing in libm-test.inc to
ALL_RM_TEST is also included.

Tested x86_64 and x86; also spot-checked results of nextafter tests
for powerpc32 and mips64 to test the ldbl-128ibm and ldbl-128
changes.  (The m68k change is untested.)

	[BZ #16677]
	* math/s_nextafter.c (__nextafter): Do not return value from
	overflowing computation.
	* sysdeps/i386/fpu/s_nextafterl.c (__nextafterl): Likewise.
	* sysdeps/ieee754/flt-32/s_nextafterf.c (__nextafterf): Likewise.
	* sysdeps/ieee754/ldbl-128/s_nextafterl.c (__nextafterl):
	Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c (__nextafterl):
	Likewise.
	* sysdeps/m68k/m680x0/fpu/s_nextafterl.c (__nextafterl): Likewise.
	* math/libm-test.inc (nextafter_test): Use ALL_RM_TEST.
Fix MIPS libc_feresetround*_ctx to preserve exceptions.

Testing on mips64 showed missing underflow exceptions (from exp, for
example) in non-default rounding modes, caused by
libc_feresetround*_ctx wrongly restoring a saved environment without
preserving exceptions, when that's only valid for the _noex variants.
(I don't know why Steve didn't see this in his testing.)  This patch
fixes this by using libc_feupdateenv_mips_ctx for the relevant macros
and removing the problem definitions.

The problem definitions aren't suitable for the _noex macros either
because they only discard exceptions in non-default rounding modes,
and while for some uses of *_noex/*_NOEX it doesn't matter whether
exceptions are discarded, dbl-64/e_remainder.c requires
SET_RESTORE_ROUND_NOEX to cause exceptions to be discarded.  I think
the accumulated set of macros / functions for optimized exception /
rounding mode handling could do with a careful review by now, and
possible refactoring, and at least one new feature (extracting the
saved rounding mode from an environment / context variable - see
dbl-64/e_sqrt.c for a case where this could be used).

Tested mips64.

	* sysdeps/mips/math_private.h [__mips_hard_float]
	(libc_feresetround_ctx): Define to libc_feupdateenv_mips_ctx not
	libc_feresetround_mips_ctx.
	[__mips_hard_float] (libc_feresetroundf_ctx): Likewise.
	[__mips_hard_float] (libc_feresetroundl_ctx): Likewise.
	[__mips_hard_float] (libc_feresetround_mips_ctx): Remove.