buildlist invocations are slow and can occur in parallel since the
underlying program obtains a lock on the modified file.
Moving the XPT-related buildlist invocation from the serial libs tier to
the parallel misc tier decreased my no-op build time on OS X from 43.5s
to 37.0s.
--HG--
extra : rebase_source : 7d274024c401b1ecfbc771424a69eb487808fcbf