kbuild: clang: Support building UM with SUBARCH=i386

The UM builds distinguish i386 from x86_64 via SUBARCH, but we don't
support building i386 directly with Clang. To make SUBARCH work for
i386 UM, we need to explicitly test for it.

This lets me run i386 KUnit tests with Clang:

$ ./tools/testing/kunit/kunit.py run \
	--make_options LLVM=1 \
	--make_options SUBARCH=i386
...

Fixes: c7500c1b ("um: Allow builds with Clang")
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250304162124.it.785-kees@kernel.org


Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>