[libunwind] fix unwinding from signal handler

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.

v2: move the increment from DwarfInstructions<A, R>::stepWithDwarf()
into the UnwindCursor<A, R>::setInfoBasedOnIPRegister() to avoid
exposing posslibly unaligned IP (also note, that this matches with gcc
implementation as well)

Refs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26208

libunwind/src/UnwindCursor.hpp

@@ -2589,6 +2589,12 @@ void UnwindCursor<A, R>::setInfoBasedOnIPRegister(bool isReturnAddress) {
     --pc;
 #endif
 
+  // In case of this is frame of signal handler, 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.
+  if (_isSignalFrame)
+    ++pc;
+
   // Ask address space object to find unwind sections for this pc.
   UnwindInfoSections sects;
   if (_addressSpace.findUnwindSections(pc, sects)) {