checkpatch: add option to not force /* */ for SPDX

checkpatch: add option to not force /* */ for SPDX

Add option --spdx-cxx-comments to not force C comments (/* */) for SPDX,
but allow also C++ comments (//).

As documented in aa19a176df95d6, this is required for some old toolchains
still have older assembler tools which cannot handle C++ style comments.
This avoids forcing this for projects which vendored checkpatch.pl (e.g.
LTP or u-boot).

Link: https://lore.kernel.org/20260421211408.383972-2-pvorel@suse.cz
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>