AltiVec assist for subnormal floats in vectors

The old CPU in a macppc traps AltiVec instructions when they encounter
denormal or subnormal floats. Emulate most of them. They operate on
vectors of

AltiVec assist for subnormal floats in vectors

The old CPU in a macppc traps AltiVec instructions when they encounter
denormal or subnormal floats. Emulate most of them. They operate on
vectors of 4 single-precision floats. The emulations either use
scalar operations (so vmaddfp becomes 4 of fmadds) or a formula (like
vrsqrtefp's 1 / sqrt(b) = 1 / sqrt(b * 2**126) * 2**63).

I am forgetting to emulate some instructions (at least vrfin, vrfiz,
vrfip, vrfim). If I don't emulate it, it will still cause SIGFPE.

Mac OS never emulated these instructions, but set AltiVec's "non-Java"
NJ bit (which changes all subnormal floats to zero). FreeBSD also
sets NJ; NetBSD does SIGFPE; Linux emulates them. The POWER9 running
OpenBSD/powerpc64 does them in hardware (without trapping).

ok kettenis@ miod@

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