introduce generic sys_sigaltstack(), switch x86 and um to it

Conditional on CONFIG_GENERIC_SIGALTSTACK; architectures that do not
select it are completely unaffected

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

arch/um/kernel/signal.c

@@ -132,8 +132,3 @@ long sys_sigsuspend(int history0, int history1, old_sigset_t mask)
 	siginitset(&blocked, mask);
 	return sigsuspend(&blocked);
 }
-
-long sys_sigaltstack(const stack_t __user *uss, stack_t __user *uoss)
-{
-	return do_sigaltstack(uss, uoss, PT_REGS_SP(&current->thread.regs));
-}