KVM: PPC: Book3S HV: Allow negative offsets to real-mode hcall handlers

The table of offsets to real-mode hcall handlers in book3s_hv_rmhandlers.S
can contain negative values, if some of the handlers end up before the
table in the vmlinux binary.  Thus we need to use a sign-extending load
to read the values in the table rather than a zero-extending load.
Without this, the host crashes when the guest does one of the hcalls
with negative offsets, due to jumping to a bogus address.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>