Apply -ffreestanding for the musl source code

Apply -ffreestanding for the musl source code

This is the corresponding commit for unikraft/unikraft#740. This
prevents a self dependency caused by compiler optimizations, because the
libc implementation cannot rely on underlying presence of a libc.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #35