x86,objtool: Separate unret validation from unwind hints

The ENTRY unwind hint type is serving double duty as both an empty
unwind hint and an unret validation annotation.

Unret validation is unrelated to unwinding. Separate it out into its own
annotation.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/ff7448d492ea21b86d8a90264b105fbd0d751077.1677683419.git.jpoimboe@kernel.org