objtool: Keep track of retpoline call sites

Provide infrastructure for architectures to rewrite/augment compiler
generated retpoline calls. Similar to what we do for static_call()s,
keep track of the instructions that are retpoline calls.

Use the same list_head, since a retpoline call cannot also be a
static_call.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/20210326151300.130805730@infradead.org