[libunwind] Fix exposure of UNW_AARCH64_RA_SIGN_STATE through _Unwind_GetGR (#209947)

The pseudo register was previously hacked in under kRegisterIsUndefined, leaving it in a state that was only accessible from within the unwinder itself.
To fix this, allowing external access to the state (and therefore tests!), we invent a new register location kRegisterIsPseudo to record the additional state that the register has been defined by a .cfi_negate_ra_state opcode having been executed.

GitOrigin-RevId: a01ff35d0375d8a2a558c44ef73583da58dfa505