runtime: set StackSystem to 1024 on Darwin/ARM.

Darwin/ARM has a bug in sigaltstack(), signal handler won't use
alternative stack no matter how you set up sigaltstack().

This will make os/signal test a bit more stable, but still not
100% stable. Also, TestSmhasherTwoNonzero in runtime will fail
with index out of range panic on system stack in mHeap_Grow after
running for ~123 seconds on an iPad3.

A test program is as follows:
#include <sys/types.h>
#include <signal.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

volatile char *volatile p;
void sighand(int sig) {
	volatile char _;
	p = &_;
}
const int size = 32<<10;
int main() {
	volatile char _;
	stack_t stk, stk2;
	struct sigaction sa;
	sa.sa_handler = sighand;
	sigemptyset(&sa.sa_mask);
	sa.sa_flags = SA_ONSTACK;
	stk.ss_sp = malloc(size);
	stk.ss_size = size;
	stk.ss_flags = 0;
	int r = sigaltstack(&stk, NULL);
	if (r) {
		perror("sigaltstack:set");
		return 1;
	}
	r = sigaltstack(NULL, &stk2);
	if (r) {
		perror("sigaltstack:get");
		return 2;
	}
	printf("get: sp: %p, size: %lld, flags: %d\n", stk2.ss_sp, (long long) stk2.ss_size, stk2.ss_flags);
	if (sigaction(SIGALRM, &sa, NULL)) {
		perror("sigaction");
		return 2;
	}
	alarm(1);
	sleep(10);
	printf("sighand stk:%p ptr:%p main stk:%p\n", p, stk.ss_sp, &_);
	if (((unsigned long long)((char *)p - (char *)stk.ss_sp)) < stk.ss_size)
		printf("Running on alt signal stack\n");
	else
		printf("Still runs on main stack, sigaltstack bug!\n");
	return 0;
}