pr18841 tests on powerpc64

The PR18841 test does cross-module calls from within an ifunc
resolver, which is nasty, and not supported in general since the
called function may not be relocated.  In this case the called
function (zoo) is just a stub so doesn't need relocating, but on ppc64
the function descriptor for zoo in the executable won't be relocated
at the time the shared library ifunc resolver runs.  That means the
test will fail if your compiler generates PIEs by default.

	PR 18841
	* testsuite/ld-ifunc/ifunc.exp: Run pr18841 tests non-pie.