x86/smpboot: Mark native_play_dead() as __noreturn

x86/smpboot: Mark native_play_dead() as __noreturn

native_play_dead() ends by calling the non-returning function
hlt_play_dead() and therefore also never returns.

The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
unconditionally calls BUG() and does not return either.

Add the __noreturn attribute to both function definitions and their
declaration to document this behavior and to potentially improve
compiler optimizations.

Remove the obsolete comment, and add native_play_dead() to the objtool's
list of __noreturn functions.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20251027155107.183136-1-thorsten.blum@linux.dev
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>