target/ppc: Improve syscall exception logging

system calls (at least in Linux) use registers r3-r8 for inputs, so
include those registers in the dump.

This also adds a mode for PAPR hcalls, which have a different calling
convention.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20200317054918.199161-1-npiggin@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>