Add linker + executable tests

[m4b]

I concurrently check on my local machine to make sure the object file is linkable (and runnable!) by both ld and gold for elf, and the OS X system linker on OS X.

It will be interesting to get a testing framework like this setup on Travis or whatever.

At some future date we can add lld as another linker target.

Also, unrelatedly, in the process of writing this I learned it’s shockingly easy to segfault a linker, which is honestly a little bit scary.

[m4b]

This is done primarily in 1e64b78