x86,objtool: Explicitly mark idtentry_body()s tail REACHABLE

Objtool can figure out that some \cfunc()s are noreturn and then
complains about certain instances having unreachable tails:

  vmlinux.o: warning: objtool: asm_exc_xen_unknown_trap()+0x16: unreachable instruction

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220408094718.441854969@infradead.org