musl upgrade - continuation

[Waldek Kozaczuk]

Even though the latest commit has upgraded musl from 0.9.12 to 1.1.24, there is still some work left.

First, it would be nice to upgrade musl to the latest version - 1.2.1 - which is only 2 minor versions ahead of 1.1.24 so hopefully, the effort should be way smaller than the last one.

Secondly, there are still some outstanding libc modules that need extra touches:

• libc/arch - https://github.com/cloudius-systems/osv/blob/master/libc/arch/README.md; finishing should improve aarch64 support even more (more about it in a separate email)
• libc/network - https://github.com/cloudius-systems/osv/blob/master/libc/network/README.md; this should be tackled when merging ipv6 branch into master which I hope to get to some time this year
• libc/locale - https://github.com/cloudius-systems/osv/blob/master/libc/locale/README.md
• libc/stdio - https://github.com/cloudius-systems/osv/blob/master/libc/stdio/README.md

Also even though I have tested this upgrade pretty thoroughly against ~ 30 real apps and added bunch of new unit tests, I am not 100% confident no new bugs have sneaked in. For example, I missed to test it against the erlang runtime. If anybody could help testing it, it would be great. 

If you have any ideas/suggestions please let me know.

Waldek