musllibc_set_environment_flags: use link groups

libgcc has dependencies on libc and all other libraries have
dependencies on libgcc. We use link groups to describe these circular
dependencies. The existing method this replaces is insufficient when
encountering symbol dependencies that are too long.

CMake: Add Findmusllibc.cmake module

This adds support for importing this project via
`find_package(musllibc)` if the project exists in the CMAKE_MODULE_PATH.

musllibc_set_environment_flags is a macro that will set build flags and
compilation rules in the current directory scope for building artifacts
that use musllibc.

musllibc_import_library adds the musllibc CMakeLists.txt file to the
current build directory.

musllibc_setup_build_environment_with_sel4runtime imports muslibc and
the sel4runtime project and also calls musllibc_set_environment_flags.