[patch] Add gdb.tar rule to top-level Makefile.in

[Andrew Cagney]

> Once I've re-checked that the patch doesn't affect the binutils snapshot process, I'll check it in (gdb 5.2 branch and src mainline).

Done. Found two things:

- BINUTILS snapshot process relies on GNU make
- the new do-bz2 rule was (re-)creating the tar ball created by do-tar.

Neither were affecting GDB.  I've fixed both and committed the change.

Andrew