[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
