gdb: Add a dependency between gdb and libbacktrace

GDB is going to start using libbacktrace, so add a build dependency
between the two modules.  This change needs to be added into the GCC
toplevel files, and then back-ported to the binutils-gdb repository.

2021-08-31  Andrew Burgess  <andrew.burgess@embecosm.com>

ChangeLog:

	* Makefile.def: Add all-gdb dependency on all-libbacktrace.
	* Makefile.in: Regenerate.