tools/lib/subcmd: Replace NORETURN usage with __noreturn

NORETURN is redundant with __noreturn, just use the latter.

Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lore.kernel.org/r/c7c83d1e6b3d2b0c3e65dd3790c22c772d3b2527.1681853186.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>