Support musl-libc based linux RIDs (dotnet/corefx#28560)

* Support musl-libc based linux RIDs

- Add `linux-musl` RID
- Set `alpine`'s parent to `linux-musl`
- Add `alpine.3.7` to the graph
- Fix up `android` to inherit from `unix` while we are here

Fixes https://github.com/dotnet/core-setup/issues/3817

* Change linux-musl to be parented to linux.

* Reparent android RID to linux.


Commit migrated from dotnet/corefx@81ba628