Add feholdexcept inline in generic math_private.h.

Continuing the process of improving and cleaning up the handling of
configurations lacking support for floating-point exceptions and
rounding modes, this patch adds trivial inline definitions of
feholdexcept and __feholdexcept to the set of inlines for such
configurations in math_private.h.  These inlines were missing from the
tile version used as a basis for the previous inlines, despite a few
such function calls ending up in libm.so.

Tested with build-many-glibcs.py.  As expected, installed stripped
shared libraries are unchanged for architectures supporting exceptions
and rounding modes, but changed for architectures lacking such
support.

	* sysdeps/generic/math_private.h
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (feholdexcept):
	New inline function.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (__feholdexcept):
	Likewise.