Switch to libunwind to remove dependency on libgcc for musl

The libgcc dependency is only used for _Unwind* symbols, remove
it now that libc_musl.so embeds libunwind.a.

Bug: 215802826
Test: m USE_HOST_MUSL=true host-native
Change-Id: I1ad4c1435fc0090c6df2d50d542d2203dfb94d7b