exec: Add KUnit test for bprm_stack_limits()

Since bprm_stack_limits() operates with very limited side-effects, add
it as the first exec.c KUnit test. Add to Kconfig and adjust MAINTAINERS
file to include it.

Tested on 64-bit UML:
$ tools/testing/kunit/kunit.py run exec

Link: https://lore.kernel.org/lkml/20240520021615.741800-1-keescook@chromium.org/


Signed-off-by: Kees Cook <kees@kernel.org>