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
LLD version 8 has added support for armv6m:
https://reviews.llvm.org/D55555
This means we can use LLD instead of arm-none-eabi-ld, eliminating our
dependency on GNU binutils.
There are small differences in code size, but never more than a few
bytes.
This commit does a few things:
* remove the -8 suffix on macOS, where it is not necessary
* add smoke tests for compiling wasm files on Linux and macOS
