RFR: Add limited support for -musl to config.sub

[Mikael Vidstedt]

The version of autoconfig-config.sub checked into the JDK repo is old enough to not support/handle musl. This change adds (very) limited support for handling musl tuples. Specifically, it *only* handles the exact tuple 'x86_64-unknown-linux-musl' and handles it by returning the exact same tuple string.

This clearly needs to be made more flexible going forward, and I’m following up on options to do so. Meanwhile, this at least works well enough to get something building.

http://cr.openjdk.java.net/~mikael/webrevs/portola/configsub/webrev.00/webrev/

Cheers,
Mikael