[mach-o] Sort GOT entries by name to make links reproducible

The GOT slots were being laid out in a random order by the GOTPass which
caused randomness in the output file.

Note: With this change lld now bootstraps on darwin.  That is:
1) link lld using system linker to make lld.1
2) link lld using lld.1 to make lld.2
3) link lld using lld.2 to make lld.3
Now lld.2 and lld.3 are identical.