Add extra libgcc.a only for unbundled build.

We can save some space for platform build with this.
Previously an extra libgcc.a was put before libc.so so libraries built
against new platform can run on old platforms. This is needed only for
unbundled build.

Bug: 8805022
Bug: 8247455

Change-Id: I3c3e0092b06ec3804659d5cae048d29e0d2bf7ad