compiler, runtime: optimize zero-sized allocations

Instead of referring to an unused global, use a constant value. This is
safe even when using `-gc=none` (since no actual memory gets allocated)
which wasn't the case before. It should also reduce binary size by a few
bytes for most programs.

src/runtime/os_windows.go

@@ -4,6 +4,8 @@ import "unsafe"
 
 const GOOS = "windows"
 
+const zeroSizeAllocPtr uintptr = 16 // part of the first protected page
+
 //export GetModuleHandleExA
 func _GetModuleHandleExA(dwFlags uint32, lpModuleName unsafe.Pointer, phModule **exeHeader) bool