ppc: testsuite: require target effectively [PR104253]

ppc: testsuite: require target effectively [PR104253]

The testcase was missing dg- before require-effective-target.

While at that, I'm also pruning the excess-error warning I got when
the test failed to be disabled because of the above.  I suppose it
might be useful for some target variants.

for gcc/testsuite/ChangeLog

	PR target/104253
	* gcc.target/powerpc/pr104253.c: Add missing dg- before
	require-effective-target.  Prune warning about -mfloat128
	possibly not being fully supported.