[libunwind] fix unwinding from signal handler (#92291)

In case of this is frame of signal handler, the IP should be
incremented, because the IP saved in the signal handler points to first
non-executed instruction, while FDE/CIE expects IP to be after the first
non-executed instruction.

Refs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26208
NOKEYCHECK=True
GitOrigin-RevId: 7b604cdf75fd1c741a15138684ea0e98dca5e46f