enable ability to remove TLS from musl

[gparmer]

Problem: ``musluses TLS to provide thepthread_` api for accessing TLS. We'd like to avoid the dependency and associated issues with TLS if we can avoid them.

Solution: If musl is loaded into a component that also provides scheduling services, then we should change musl (likely using weak magic) to call out to the sl explicitly to get the TLS region. This will have the side-effect of programmatically bombing out if TLS isn't set up for a thread, rather than causing a random fault.b

Assigned this to quite a few people as I don't know who is going to run into the prickly edges first.