Add synthetic IntegerDivideByZero exception

The ROM routine __divsi3 is called by code whenever a division is needed, because there is no divide unit on the ESP8266 core. When the divide routine in ROM hits a div-by-zero case, it jumpt to an ILL(egal instruction) at a fixed address which causes a HW exception 0 (IllegalInsnException). In the postmortem dump, when an ILL exception is detected at this address in ROM, convert it to a DivByZeroException for printout (6). Divde by zero errors now print as follows: ```` --------------- CUT HERE FOR EXCEPTION DECODER --------------- Exception (6): epc1=0x4000dce5 epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000 >>>stack>>> ... <<<stack<<< --------------- CUT HERE FOR EXCEPTION DECODER --------------- ```` And will decode as follows: ```` Exception 6: IntegerDivideByZero: QUOS, QUOU, REMS, or REMU divisor operand is zero PC: 0x4000dce5 EXCVADDR: 0x00000000 Decoding stack results ... ````

cores/esp8266/core_esp8266_postmortem.cpp

@@ -144,8 +144,11 @@ void __wrap_system_restart_local() {
  ets_printf_P(PSTR("\nAbort called\n"));
  }
  else if (rst_info.reason == REASON_EXCEPTION_RST) {
+ // The GCC divide routine in ROM jumps to the address below and executes ILL (00 00 00) on div-by-zero
+ // In that case, print the exception as (6) which is IntegerDivZero
+ bool div_zero = (rst_info.exccause == 0) && (rst_info.epc1 == 0x4000dce5);
  ets_printf_P(PSTR("\nException (%d):\nepc1=0x%08x epc2=0x%08x epc3=0x%08x excvaddr=0x%08x depc=0x%08x\n"),
- rst_info.exccause, rst_info.epc1, rst_info.epc2, rst_info.epc3, rst_info.excvaddr, rst_info.depc);
+ div_zero ? 6 : rst_info.exccause, rst_info.epc1, rst_info.epc2, rst_info.epc3, rst_info.excvaddr, rst_info.depc);
  }
  else if (rst_info.reason == REASON_SOFT_WDT_RST) {
  ets_printf_P(PSTR("\nSoft WDT reset\n"));