gn2bp: do not skip //buildtools and buildflag generation

We (for now at least) depend on libc++ inside //buildtools, so the
dependency should not be skipped.
Similarly, perfetto uses a hack to create the buildflags target. This
will not work for us.

Test: run script
Change-Id: I63c8cf4fd1c95f6336e905e02cdf66cec6dc85e9