Builder remove *.a before building a new one

This is noticed by building libportable.a when quite a few
functions were removed.  Rebuilding them doesn't shrink
size because the old *obj are still in *.a, and "ar crs"
only replace the one already in *.a

Change-Id: Ifcb2935e20839fac0005e3232f6b0a70d727f6f0
1 file changed