Add support for the gdb only package.

If the --gdb_only arg is specified we
install gdb to a new destination outside
of the install dir, so we can package it separately.

GNUTOOLS-15776

Change-Id: Id05d9a0ee343c59985c7f2851db434a5d5ee1c10
Reviewed-on: https://eu-gerrit-1.euhpc.arm.com/c/dsg-gnu/Internal/gnu-devtools-for-arm/+/759533


Reviewed-by: Vaseeharan Vinayagamoorthy <vaseeharan.vinayagamoorthy@arm.com>
Tested-by: Service account - used by users in DSG GNU team <dgboter@arm.com>