Add support for relative call instructions in reentry island

[noahd]

Currently, the fixupInstructions function only relocates relative jump offsets and nothing else. This causes segfaults when calling the reentry islands of functions that contain relative call instructions in their first 5 bytes.

If you're looking for a concrete example, check out the PKGSpaceGet* functions in CoreGraphics (they're only used in WindowServer and aren't exported, FYI):

---

There are other types of instructions with relative offsets that also cause segfaults in reentry islands*, but I honestly don't feel adept enough at the moment to write a general solution, so I left it at that.

* Compiling test_mach_override.cp with clang yields crashes, for instance. I'm assuming that happens because the nops in localFunction() get optimized away and the relative load instruction for __FUNCTION__ is copied as-is.