powerpc64le: fix boot when using QEMU PowerNV

When using QEMU PowerNV with latest op-build release (v2.7), its
kexec transfers control to FreeBSD kernel in BE mode, causing an
instant exception on LE kernels. Make kboot able to detect and
swap endian to fix this.

Reviewed by: imp
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D33104