top-level: merge Makefile.def patches from binutils-gdb repository

This commit back-ports two patches to Makefile.def from the
binutils-gdb repository, these patches were committed over there
without first being merged in to the gcc repository.

These commits all relate to dependencies for binutils-gdb modules, so
should have no impact on gcc, I tested a gcc build/install on x86-64
GNU/Linux, and everything looked OK.

The two patches being backported are binutils-gdb commits:

  commit ba4d88ad892fe29c6ca7938c8861f8edef5f7a3f (gdb-gnulib-issues)
  Date:   Mon Oct 12 16:04:32 2020 +0100

      gdb/gdbserver: add dependencies for distclean-gnulib

And

  commit 755ba58ebef02e1be9fc6770d00243ba6ed0223c
  Date:   Thu Mar 18 12:37:52 2021 +0000

      Add install dependencies for ld -> bfd and libctf -> bfd

2021-09-07  Andrew Burgess  <andrew.burgess@embecosm.com>

	* Makefile.def: Back-port commits ba4d88ad892f and
	755ba58ebef0 from binutils-gdb repository.
	* Makefile.in: Regenerated.