Add #include <GDBStub.h> to the sketch
Upload the sketch
Redirect serial port to TCP port:
1 tcp_serial_redirect.py -p /dev/tty.SLAB_USBtoUART -b 115200 --spy -P 9980 --rts=0 --dtr=0

Change port and baud rate as necessary. This command requires python and pyserial.
Observe serial output:
1 nc localhost 9980
Once crash happens, close nc and start gdb:
1 xtensa-lx106-elf-gdb /path/to/Sketch.cpp.elf -ex "target remote :9980"

Or, using the provided gdbcmds file:
1 xtensa-lx106-elf-gdb /path/to/Sketch.cpp.elf -x gdbcmds
Use gdb to inspect program state at the point of an exception.

Tips and tricks

To halt the target when software WDT fires, add
1 ((int*)0) = 0;

at the top of __wrap_system_restart_local in core_esp8266_postmortem.c.

License

Espressif MIT License. See License file.