Unlist libunwind as a critical component

Virtually nothing uses libunwind, and no one should start using it, so
it's unlikely to be critical.

(Code should use libunwindstack.so to collect backtraces, and it should
use libc.so's _Unwind_* APIs for C++ exception handling unwinding.)

Test: build Android
Change-Id: I81882a4634ff4d397abe8a7cf5ff15a22a3eed2f