linux-user: Support for restarting system calls for PPC targets

Update the PPC main loop code:
 * on TARGET_ERESTARTSYS, wind guest PC backwards to repeat syscall insn

(We already handle TARGET_QEMU_ESIGRETURN.)

Signed-off-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Message-id: 1441497448-32489-8-git-send-email-T.E.Baldwin99@members.leeds.ac.uk
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: tweak commit message; drop TARGET_USE_ERESTARTSYS define]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>