Allows the altivec packetmath tests to pass. There were a few issues:
- `pstoreu` was missing MSQ on `_BIG_ENDIAN` systems
- `cmp_*` didn't properly handle conversion of bool flags (0x7FC instead
of 0xFFFF)
- `pfrexp` needed to set the `exponent` argument.
Related to !370, #2128
cc: @ChipKerchner @pdrocaldeira
Tested on `_BIG_ENDIAN` running on QEMU with VSX. Couldn't figure out build
flags to get it to work for little endian.
1615a27993