Previously, Zig ignored -lgcc_s with a warning that this dependency is
redundant because it is satisfied by compiler-rt. However, sfackler
pointed out that it also provides exception handling functions. So if
Zig sees -lgcc_s on the linker line, it needs to fulfill this dependency
with libunwind.

I also made link_libc inferred to be on if libunwind is linked since
libunwind depends on libc.