mk: Add support for musl-based builds

This commit adds support to the makefiles, configuration script, and build
system to understand MUSL. This is broken up into a few parts:

* Any target of the form `*-musl` requires the `--musl-root` option to
  `./configure` which will indicate the root of the MUSL installation. It is
  also expected that there is a libunwind build inside of that installation
  built against that MUSL.

* Objects from MUSL are copied into the build tree for Rust to be statically
  linked into the appropriate Rust library.

* Objects for binary startup and shutdown are included in each Rust installation
  by default for MUSL. This requires MUSL to only be installed on the machine
  compiling rust. Only a linker will be necessary for compiling against MUSL on
  a target machine.

Eventually a MUSL and/or libunwind build may be integrated by default into the
build but for now they are just always assumed to exist externally.