[libunwind][aarch64] Fix auth on PAuth_LR'd frames (#206814)

The RA_SIGN_STATE pseudo register is a three-state enum, so it should be
encoded as one in the unwinder's representation. This fixes a bug on the
authentication path, which assumed "is signed with PC" was a subset of
"is signed".

GitOrigin-RevId: 4d87e55d8a41872ef40079866d0527d8a1381c57