switch mips to generic rt_sigsuspend(), make it unconditional

mips was the last architecture not using the generic variant.
Both native and compat variants switched to generic, which is
made unconditional now.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/signal.c

@@ -3562,7 +3562,6 @@ int sigsuspend(sigset_t *set)
 	return -ERESTARTNOHAND;
 }
 
-#ifdef __ARCH_WANT_SYS_RT_SIGSUSPEND
 /**
  *  sys_rt_sigsuspend - replace the signal mask for a value with the
  *	@unewset value until a signal is received
@@ -3603,7 +3602,6 @@ COMPAT_SYSCALL_DEFINE2(rt_sigsuspend, compat_sigset_t __user *, unewset, compat_
 #endif
 }
 #endif
-#endif
 
 #ifdef CONFIG_OLD_SIGSUSPEND
 SYSCALL_DEFINE1(sigsuspend, old_sigset_t, mask)