fix: add `libc.so` fallback for musl systems to the ctypes impl

fix: add `libc.so` fallback for musl systems to the ctypes impl

Add a fallback to `libc.so` library name to fix loading the ctypes
implementation on musl systems.  On musl, `find_library("c")` does
not work (the problem has been reported to CPython in 2014, and has not
been resolved yet), causing the module to fail on `assert libcname`.
Instead, add a fallback to using `libc.so` and let ctypes raise
an exception if such a library does not exist.