ppc64: handle exception 0x1500 (soft patch)

This change adds a hypervisor trap handler for exception 0x1500 (soft patch),
normalizing all VSX registers and returning.
This avoids a kernel panic due to unknown exception.

Change made with the collaboration of leonardo.bianconi_eldorado.org.br,
that found out that this is a hypervisor exception and not a supervisor one,
and fixed this in the code.

Reviewed by: jhibbits, sbruno
Differential Revision: https://reviews.freebsd.org/D17806