tools/x86/kcpuid: Simplify usage() handling

tools/x86/kcpuid: Simplify usage() handling

Refactor usage() to accept an exit code parameter and exit the program
after usage output.  This streamlines its callers' code paths.

Remove the "Invalid option" error message since getopt_long(3) already
emits a similar message by default.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20250324142042.29010-4-darwi@linutronix.de