x86,objtool: Move the ASM_REACHABLE annotation to objtool.h

Because we need a variant for .S files too.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yi9gOW9f1GGwwUD6@hirez.programming.kicks-ass.net