zfs/powerpc64: Fix big-endian powerpc64 asm

The powerpc asm from openzfs assumes that big-endian is always ELFv1 and
ELFv2 is always little-endian, while FreeBSD uses ELFv2 everywhere. Add
the necessary bits to the checksum asm to work on big-endian ELFv2.

This was also submitted upstream as PR#14779.

Tested by: dbaio