powerpc64: Support ELFv2 asm on Big Endian

FreeBSD/powerpc64 is all ELFv2 since FreeBSD 13, even big endian. The
existing sha256 and sha512 asm code assumes that BE is all ELFv1, and LE
is ELFv2. Minor changes to add ELFv2 in the BE side gets this working
correctly on FreeBSD with latest OpenZFS import.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Justin Hibbits <chmeeedalf@gmail.com>
Closes #14779