Fix powerpc nearbyint wrongly clearing "inexact" and leaving traps disabled (bug 19228).

Similar to bug 15491 recently fixed for x86_64 / x86, the powerpc
(both powerpc32 and powerpc64) hard-float implementations of
nearbyintf and nearbyint wrongly clear an "inexact" exception that was
raised before the function was called; this shows up as failure of the
test math/test-nearbyint-except added when that bug was fixed.  They
also wrongly leave traps on "inexact" disabled if they were enabled
before the function was called.

This patch fixes the bugs similar to how the x86 bug was fixed: saving
and restoring the whole floating-point state, both to restore the
original "inexact" flag state and to restore the original state of
whether traps on "inexact" were enabled.  Because there's a convenient
point in the powerpc implementations to save state after any sNaN
arguments will have raised "invalid" but before "inexact" traps need
to be disabled, no special handling for "invalid" is needed as in the
x86 version.

Tested for powerpc64 and powerpc32, where it fixes the
math/test-nearbyint-except failure as well as fixing the new test
math/test-nearbyint-except-2 added by this patch.  Also tested for
x86_64 and x86 that the new test passes.

If powerpc experts see a more efficient way of doing this
(e.g. instruction positioning that's better for pipelines on typical
processors) then of course followups optimizing the fix are welcome.

	[BZ #19228]
	* sysdeps/powerpc/powerpc32/fpu/s_nearbyint.S (__nearbyint): Save
	and restore full floating-point state.
	* sysdeps/powerpc/powerpc32/fpu/s_nearbyintf.S (__nearbyintf):
	Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_nearbyint.S (__nearbyint):
	Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_nearbyintf.S (__nearbyintf):
	Likewise.
	* math/test-nearbyint-except-2.c: New file.
	* math/Makefile (tests): Add test-nearbyint-except-2.

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