powerpc64/pseries: Fix hypervisor call with extra arguments

Some hypervisor calls, such as H_SEND_LOGICAL_LAN, take more arguments than
are traditionally passed in registers. The HCALL ABI will accept these
arguments in r11 and r12. With ELFv2 ABI, these arguments are 2
double-words lower than ELFv1 ABI, as two double-words in the stack frame
are no longer used, and therefore removed from the frame. Fix the offsets
for loading the registers for the HCALL. This fixes the phyp_llan driver
with ELFv2 kernel.

Submitted by: alfredo.junior_eldorado.org.br
Differential Revision: https://reviews.freebsd.org/D20008