Add musl libc submodule

Since musl is already incorporated into the build system (see for
example configure.ac, Makefile.in), this patch adds a submodule for
musl based on the current v1.2.2 release.
The GitHub Actions configurations have also been updated to include
the musl based toolchain.

Signed-off-by: Florian Hofhammer <florian.hofhammer@fhofhammer.de>