powerpc/64s: syscall real mode entry use mtmsrd rather than rfid

Have the real mode system call entry handler branch to the kernel
0xc000... address and then use mtmsrd to enable the MMU, rather than use
SRRs and rfid.

Commit 8729c26e

 ("powerpc/64s/exception: Move real to virt switch
into the common handler") implemented this style of real mode entry for
other interrupt handlers, so this brings system calls into line with
them, which is the main motivcation for the change.

This tends to be slightly faster due to avoiding the mtsprs, and it also
does not clobber the SRR registers, which becomes important in a
subsequent change. The real mode entry points don't tend to be too
important for performance these days, but it is possible for a
hypervisor to run guests in AIL=0 mode for certian reasons.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210208063326.331502-1-npiggin@gmail.com