[libunwind] Fix an uninitialized read of __ra_sign_state (#205152)

The Arm DWARF spec defines UNW_AARCH64_RA_SIGN_STATE as being zeroed
until the first .cfi_negate_ra_state / .cfi_set_ra_state [1]. The GPRs
struct containing __ra_sign_state is memcpy'd directly from the
unw_context_t, which in turn is initialized by __unw_getcontext. Since
it is a pseudo register, there is no corresponding state to restore in
__unw_resume.

https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#44call-frame-instructions

GitOrigin-RevId: e67b31a00e72ebb1b3a782c6bfcad9b51f173521