runtime: Don't ask mmap for wrapping memory.

From-SVN: r180732

libgo/runtime/malloc.goc

@@ -358,6 +358,8 @@ runtime_mallocinit(void)
 		// away from the running binary image and then round up
 		// to a MB boundary.
 		want = (byte*)(((uintptr)end + (1<<18) + (1<<20) - 1)&~((1<<20)-1));
+		if(0xffffffff - (uintptr)want <= bitmap_size + arena_size)
+		  want = 0;
 		p = runtime_SysReserve(want, bitmap_size + arena_size);
 		if(p == nil)
 			runtime_throw("runtime: cannot reserve arena virtual address space");