Filter out unnecessary directories from BR archives

BR doesn't play well with our new unified binutils-gdb reporsiry because it
contains both projects at once, however ARC-specific files are only in
relevant branches. As a result GDB cannot be built from binutils branch and
vice versa. That causes issue with BR which just calls make and tries to
build everything, which is results in build failure, since it is trying to
build bintutils/gdb which is not aware of ARC at all. To avoid this problem
we need to filter out extra directories when creating tars for BR.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>