When ripping out Qualcomm's EL2 on the x13s, calls into the Secure Monitor

When ripping out Qualcomm's EL2 on the x13s, calls into the Secure Monitor
make the machine crash.  As it turns out the SMC calling convention specifies
that further registers up to and including x18 might need to be caller-saved,
which we seem to have to follow now.

ok kettenis@