Fix MIPS libc_feresetround*_ctx to preserve exceptions.

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.