Try to workaround iOS crash, tweaks

Since it's apparently crashing iOS, let's ifdef it out.  Although, it doesn't crash it for me, and it's slower this way.  Oh well...

Also, avoid using regs that need to be saved, since there are plenty on NEON anyway.

And, I think the spurious timeouts on the buildbot are caused by some sort of stack corruption.  It's happening on Android too.  I didn't check our register saving, though, but wondering if this will fix x64 on the buildbot.

-[Unknown]