[libunwind][AArch64] Fix _Unwind_ForcedUnwind via sigreturn.

When the sigreturn trampoline is found the unw_proc_info_t.end_ip need to be set to
indicate a stack frame is found.

Reviewed By: cjdb, #libunwind, MaskRay

Differential Revision: https://reviews.llvm.org/D124522