Add regress for powerpc* AltiVec with subnormal floats

Older CPUs do an AltiVec assist trap, EXC_VECAST_G4 or EXC_VECAST_G5
in /sys/arch/powerpc/powerpc/trap.c; newer CPUs (like POWER9) don't
trap. The trap's emulation of subnormal floats should give almost the
same results as the POWER9.

This test is only for powerpc or powerpc64 with AltiVec. anton@
explained how to skip the test if we're on the wrong arch or don't
have AltiVec.

ok anton@