xfail fetestexcept test - ppc always uses fcmpu

xfail fetestexcept test - ppc always uses fcmpu

gcc.dg/torture/pr91323.c tests that a compare with NaNf doesn't set an
exception using builtin compare intrinsics, and that it does when
using regular compare operators.

That doesn't seem to be expected to work on powerpc targets.  It fails
on GNU/Linux, it's marked to be skipped on AIX, and a similar test,
gcc.dg/torture/pr93133.c, has the execution test xfailed for all of
powerpc*-*-*.

In this test, the functions that use intrinsics for the compare end up
with the same code as the one that uses compare operators, using
fcmpu, a floating compare that, unlike fcmpo, does not set the invalid
operand exception for quiet NaN.  I couldn't find any evidence that
the rs6000 backend ever outputs fcmpo.  Therefore, I'm adding the same
execution xfail marker to this test.

for  gcc/testsuite/ChangeLog

	PR target/58684
	* gcc.dg/torture/pr91323.c: Expect execution fail on
	powerpc*-*-*.