kbuild: scripts/gdb: bring the "abspath" back

Use the "abspath" call when symlinking the gdb python scripts in
scripts/gdb/linux. This call is needed to avoid broken links when
running the scripts_gdb target on a build directory located directly
under the source tree (e.g., O=builddir).

Fixes: 659bbf7e ("kbuild: scripts/gdb: Replace missed $(srctree)/$(src) w/ $(src)")
Signed-off-by: Joel Granados <j.granados@samsung.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>