Fix powerpc fesetexceptflag clearing FE_INVALID (bug 20455).

As shown by the test math/test-fexcept, the powerpc fesetexceptflag
implementation fails to clear a previously set FE_INVALID flag, when
that flag is clear in the saved exceptions and FE_INVALID is included
in the mask of flags to restore, because it fails to mask out the
sub-exceptions of FE_INVALID from the FPSCR state.  This patch fixes
the masking logic accordingly.

Tested for powerpc.

	[BZ #20455]
	* sysdeps/powerpc/fpu/fsetexcptflg.c (__fesetexceptflag): Mask out
	all FE_INVALID sub-exceptions from FPSCR when FE_INVALID specified
	to be restored.