musl libc support

[Chris Patterson]

Snapcraft uses the "gnu" arch triplet (e.g. x86_64-linux-gnu), whereas musl reuires the "musl" triplet (e.g. x86_64-linux-musl).

Users of musl libc will require a layout to wire up musl but it can be made to work with recent snapcraft, though will get an error:

This part is missing libraries that cannot be satisfied with any available stage-packages known to snapcraft: - libc.musl-x86_64.so.1

Crawling for it probably doesn't work due to the soname format?

Regardless, we should sort out the appropriate way to handle this case. There may be other gotchas.

---

layout:
  /lib/ld-musl-x86_64.so.1:
    bind-file: $SNAP/lib/x86_64-linux-musl/libc.so