patch to use target specific .gdbinit file

[Felix Lee]

"Kris Warkentin" <kewarken@qnx.com>:
> The following code will allow backend writers to define
> EXTRA_GDBINIT_FILENAME to be an alternate filename for sourcing on startup.
> For example, since we have gdb versions for 5 different CPU targets, we
> allow users to create a $HOME/.ntoCPU-gdbinit.

1, I'd rather it be a generalized name, like maybe
.gdbinit-$TARGET.  making the name something chosen by the
backend writer feels like it adds unnecessary irregularity.

2, this is going to make it more awkward to create a single
gdb that will debug multiple targets.
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