Add tests for fegetexceptflag, fesetexceptflag.

Add tests for fegetexceptflag, fesetexceptflag.

I noticed that there was no meaningful test coverage for
fegetexceptflag and fesetexceptflag (one test ensures that calls to
them compile and link, but nothing to verify they work correctly).
This patch adds tests for these functions.

fesetexceptflag is meant to set the relevant exception flag bits to
the saved state without causing enabled traps to be taken.  On some
architectures, it is not possible to set exception flag bits without
causing enabled traps to occur.  Such architectures need to define
EXCEPTION_SET_FORCES_TRAP to 1 in their math-tests.h, as is done in
this patch for powerpc.  x86 avoids needing to define this because the
traps resulting from setting exception bits don't occur until the next
floating-point operation or fwait instruction.

Tested for x86_64, x86 and powerpc.  Note that test-fexcept fails for
powerpc because of a pre-existing bug in fesetexceptflag for powerpc,
which I'll fix separately.

	* math/test-fexcept-traps.c: New file.
	* math/test-fexcept.c: Likewise.
	* math/Makefile (tests): Add test-fexcept and test-fexcept-traps.
	* sysdeps/generic/math-tests.h (EXCEPTION_SET_FORCES_TRAP): New
	macro.
	* sysdeps/powerpc/math-tests.h [!__NO_FPRS__]
	(EXCEPTION_SET_FORCES_TRAP): Likewise.