zfs: powerpc: disable kernel floating point support

Powerpc* doesn't support support floating point in the kernel yet,
so disable it on zfs for now.

This fixes "panic: altivec unavailable trap" when loading zfs.ko

Approved by: jhibbits
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D36894