[MIPS] signal: do not use save_static_function() anymore

This macro was used to save static registers before calling
sys_sigsuspend() and sys_sigreturn().

For the sys_sigreturn() case, there's no point to save them
since they have been already saved by setup_sigcontext()
before calling the signal handler.

For the sys_sigsuspend() case, I don't see any reasons...

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>