This commit makes `zig cc` match the equivalent behavior of
`zig build-exe` with regards to caching. That is - it will cache
individual .c to .o compilations (with the usual exceptions), but will
always repeat the linking process so that incremental linking has a
chance to happen.

Perhaps a future enhancement will provide a way to get the old behavior,
but I suspect this new behavior will be preferred by everyone, because
it is closer to what C compilers do. Note that the old behavior can be
obtained by switching to `zig build-exe` instead of `zig cc` and using
the `--enable-cache` parameter.

Closes #12317