kunit: tool: Build GDB scripts

Following a similar rationale as commit e4835f1d ("kunit: tool:
Build compile_commands.json"), make a common developer tool available by
default for KUnit users.

Compared to compile_commands.json, there is a little more work to be
done to build the GDB scripts. Is it enough to affect development cycle
duration? Unscientific evaluation:

rm -rf .kunit; time tools/testing/kunit/kunit.py build --kunitconfig ./lib/kunit/.kunitconfig --jobs 96

Without this patch it took 14.77s, with this patch it took 14.83. So,
although `make scripts_gdb` is pretty slow, presumably most of that is
just the overhead of running Kbuild at all, actually building the
scripts is approximately free.

Note also, to actually get the GDB scripts the user needs to enable
CONFIG_SCRIPTS_GDB, but building the scripts_gdb target without that is
still harmless.

Link: https://lore.kernel.org/r/20250121-kunit-gdb-v1-1-faedfd0653ef@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>