objtool,x86: Teach decode about LOOP* instructions

When 'discussing' control flow Masami mentioned the LOOP* instructions
and I realized objtool doesn't decode them properly.

As it turns out, these instructions are somewhat inefficient and as
such unlikely to be emitted by the compiler (a few vmlinux.o checks
can't find a single one) so this isn't critical, but still, best to
decode them properly.

Reported-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yxhd4EMKyoFoH9y4@hirez.programming.kicks-ass.net