[ARM] 3270/1: ARM EABI: fix sigreturn and rt_sigreturn

Patch from Nicolas Pitre

The signal return path consists of user code provided by the kernel.
Since a syscall is used, it has to be updated to work with EABI.

Noticed by Daniel Jacobowitz.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>