Support redirecting unexported local symbols.

[taviso]

coffsyrup cannot currently undefine local symbols, but there's no reason why it can't. The main problem is there are no relocs, so existing callers will still call the old function. There is an easy solution:

• undefine the symbol
• replace the first instruction with jmp [undefined symbol]

Now we provide a replacement, and the linker will just fix it up for us.

[taviso]

I have a patch that seems to work, but it's kinda hacky, hopefully it doesn't break anything.