The ar file format is pretty simple and can be implemented by using a Go
library. Use that instead of calling out to llvm-ar.
There are a few limitations to the used package, but that doesn't seem
to matter for our use case (linking compiler-rt for use with ld.lld):
* no index is created
* long filenames are truncated
* no support for archives bigger than 4GB
A few changes to make sure compiler-rt is correctly compiled (and
doesn't include host headers, for example).
This improves support for AVR, but it still doesn't work. Compiler-rt
itself doesn't really work for AVR either.
