kernel: Trap in log_error() when a debugger is attached.

The workflow of debugging fatal pass errors in Yosys is flawed in
three ways:
 1. Running Yosys under a debugger is sufficient for the debugger
    to catch some fatal errors (segfaults, aborts, STL exceptions)
    but not others (`log_error()`, `log_cmd_error()`). This is
    neither obvious nor easy to remember.
 2. To catch Yosys-specific fatal errors, it is necessary to set
    a breakpoint at `logv_error_with_prefix()`, or at least,
    `logv_error()`. This is neither obvious nor easy to remember,
    and GDB's autocomplete takes many seconds to suggest function
    names due to the large amount of symbols in Yosys.
 3. If a breakpoint is not set and Yosys encounters with such
    a fatal error, the process terminates. When debugging a crash
    that takes a long time to reproduce (or a nondeterministic crash)
    this can waste a significant amount of time.

To solve this problem, add a macro `YS_DEBUGTRAP` that acts as a hard
breakpoint (if available), and a macro `YS_DEBUGTRAP_IF_DEBUGGING`
that acts as a hard breakpoint only if debugger is present.

Then, use `YS_DEBUGTRAP_IF_DEBUGGING` in `logv_error_with_prefix()`
to obviate the need for a breakpoint on nearly every platform.

Co-Authored-By: Alberto Gonzalez <[email protected]>

kernel/log.cc

@@ -354,6 +354,9 @@ static void logv_error_with_prefix(const char *prefix,
 
 	if (check_expected_logs)
 		log_check_expected();
+
+	YS_DEBUGTRAP_IF_DEBUGGING;
+
 #ifdef EMSCRIPTEN
 	log_files = backup_log_files;
 	throw 0;
@@ -673,7 +676,7 @@ void log_check_expected()
 		}
 		if (item.second.current_count != item.second.expected_count) {
 			log_warn_regexes.clear();
-			log_error("Expected warning pattern '%s' found %d time(s), instead of %d time(s) !\n", 
+			log_error("Expected warning pattern '%s' found %d time(s), instead of %d time(s) !\n",
 				item.second.pattern.c_str(), item.second.current_count, item.second.expected_count);
 		}
 	}
@@ -700,7 +703,7 @@ void log_check_expected()
 				_exit(0);
 			#else
 				_Exit(0);
-			#endif			
+			#endif
 		} else {
 			display_error_log_msg = false;
 			log_warn_regexes.clear();