gdbstub, postmortem: clean up

- Move GDB stub hooks into a separate file, provide header for it
- Use syscall instruction raise user mode exception
- Remove unused code in postmortem.c

fixup

fixup

libraries/GDBStub/src/GDBStub.h

@@ -3,6 +3,4 @@
 
 // this header is intentionally left blank
 
-bool crash_for_gdb = 1;
-
 #endif //GDBSTUB_H