um: Fix -Wmissing-prototypes warnings for (rt_)sigreturn

Use SYSCALL_DEFINE0 to define (rt_)sigreturn. This will address
below -Wmissing-prototypes warnings:

arch/x86/um/signal.c:453:6: warning: no previous prototype for ‘sys_sigreturn’ [-Wmissing-prototypes]
arch/x86/um/signal.c:560:6: warning: no previous prototype for ‘sys_rt_sigreturn’ [-Wmissing-prototypes]

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>