Move fenv.h override inline functions to generic math_private.h.

Move fenv.h override inline functions to generic math_private.h.

The tile version of math_private.h defines some inline functions for
fenv.h functions, to optimize away internal calls to these functions
that do nothing given no support for floating-point exceptions and
rounding modes.  (Some functions may have error cases for invalid
arguments, but those aren't applicable to the internal calls from
within glibc.)  Other configurations lacking support for exceptions
and rounding modes lack such inline functions.  This patch moves them
to the generic math_private.h, appropriately conditioned, so that all
such configurations can benefit from the.

include/fenv.h is made to check whether there are any non-default
rounding modes; that needs to be done there, rather than later,
because get-rounding-mode.h defines values for otherwise unsupported
FE_* rounding modes.  It also gives an error for FE_TONEAREST
undefined, a case that already did not work for building the glibc
testsuite; the convention has by now been established that all
architectures need to provide a version of bits/fenv.h that at least
defines FE_TONEAREST.

Tested with build-many-glibcs.py.  As expected, installed stripped
shared libraries are unchanged for tile and for architectures
supporting exceptions and rounding modes, but changed for non-tile
architectures not supporting exceptions and rounding modes that
previously lacked this optimization (e.g. Nios II libm.so is about 1kB
smaller).

The optimization is not in fact complete (does not cover feholdexcept
/ __feholdexcept, so a few calls to those remain unnecessarily within
libm even after this patch), but that can be dealt with separately.

	* include/fenv.h [!_ISOMAC && !FE_TONEAREST]: Give #error.
	[!_ISOMAC] (FE_HAVE_ROUNDING_MODES): New macro.
	* sysdeps/generic/math_private.h
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (fegetenv): New
	inline function.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (__fegetenv):
	Likewise.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (fesetenv):
	Likewise.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (__fesetenv):
	Likewise.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (feupdateenv):
	Likewise.
	[!FE_HAVE_ROUNDING_MODES && FE_ALL_EXCEPT == 0] (__feupdateenv):
	Likewise.
	[!FE_HAVE_ROUNDING_MODES] (fegetround): Likewise.
	[!FE_HAVE_ROUNDING_MODES] (__fegetround): Likewise.
	[!FE_HAVE_ROUNDING_MODES] (fesetround): Likewise.
	[!FE_HAVE_ROUNDING_MODES] (__fesetround): Likewise.
	* sysdeps/tile/math_private.h (fegetenv): Remove inline function.
	(__fegetenv): Likewise.
	(fesetenv): Likewise.
	(__fesetenv): Likewise.
	(feupdateenv): Likewise.
	(__feupdateenv): Likewise.
	(fegetround): Likewise.
	(__fegetround): Likewise.
	(fesetround): Likewise.
	(__fesetround): Likewise.