can mark externally linked (foreign) vars as rodata

[beaumccartney]

repro below - compiles and runs

repro:

package repro

main :: proc() {
    ptr := &vm_page_mask

    fmt.println(vm_page_mask)
    ptr^ -= 1
    fmt.println(vm_page_mask)
}

foreign import mach "system:System.framework"
foreign mach {
	@(rodata) vm_page_mask: c.uintptr_t
}

import "core:fmt"
import "core:c"