exec: Avoid pathological argc, envc, and bprm->p values

Make sure nothing goes wrong with the string counters or the bprm's
belief about the stack pointer. Add checks and matching self-tests.

Take special care for !CONFIG_MMU, since argmin is not exposed there.

For 32-bit validation, 32-bit UML was used:
$ tools/testing/kunit/kunit.py run \
	--make_options CROSS_COMPILE=i686-linux-gnu- \
	--make_options SUBARCH=i386 \
	exec

For !MMU validation, m68k was used:
$ tools/testing/kunit/kunit.py run \
	--arch m68k --make_option CROSS_COMPILE=m68k-linux-gnu- \
	exec

Link: https://lore.kernel.org/r/20240520021615.741800-2-keescook@chromium.org
Link: https://lore.kernel.org/r/20240621205046.4001362-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

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