gdb, gdbserver, gdbsupport: reformat some Makefile variables, one entry per line

Reformat some variables definitions.  I think it makes them easier to
read, and it also makes diffs clearer.

Change-Id: I82f63ba0e6d0fe268eb1f1ad5ab22c3cd016ab02
Approved-By: Pedro Alves <pedro@palves.net>
4 files changed