Fix PPC_FP on non-sse4.1 code paths.

The Invalid bit on the x87 fpu is sticky, so once a single NaN goes
through the old code on CPUs without sse4.1 all future floats are
mutilated.

Patch to emulate PTEST by Fiora.

Fixes issue 7237 and issue 7510.
Merge pull request #681 from phire/non-sse4_1

Fix PPC_FP on non-sse4.1 code paths.