Make --gdb explain that it doesn't work right now
Make --gdb explain that it doesn't work right now

I'm not quite sure how to solve this especially considering
android.  I likely should use 'gdbserve' but I was not able
to get that to work reliably.

At least this makes it less confusing that --gdb doesn't
just silently fail.

R=abarth@chromium.org, hansmuller@chromium.org, ojan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/850533004

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