--exception:goto switch for deterministic exception handling (#12977)

This implements "deterministic" exception handling for Nim based on goto instead of setjmp. This means raising an exception is much cheaper than in C++'s table based implementations. Supports hard realtime systems. Default for --gc:arc and the C target because it's generally a good idea and arc is all about deterministic behavior.

Note: This implies that fatal runtime traps are not catchable anymore! This needs to be documented.

lib/system/fatal.nim

@@ -1,4 +1,19 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2019 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
 {.push profiler: off.}
+
+when defined(nimHasExceptionsQuery):
+  const gotoBasedExceptions = compileOption("exceptions", "goto")
+else:
+  const gotoBasedExceptions = false
+
 when hostOS == "standalone":
   include "$projectpath/panicoverride"
 
@@ -9,19 +24,20 @@ when hostOS == "standalone":
     rawoutput(message)
     panic(arg)
 
-elif defined(nimQuirky) and not defined(nimscript):
+elif (defined(nimQuirky) or gotoBasedExceptions) and not defined(nimscript):
   import ansi_c
 
   proc name(t: typedesc): string {.magic: "TypeTrait".}
 
   proc sysFatal(exceptn: typedesc, message, arg: string) {.inline, noreturn.} =
+    writeStackTrace()
     var buf = newStringOfCap(200)
     add(buf, "Error: unhandled exception: ")
     add(buf, message)
     add(buf, arg)
     add(buf, " [")
     add(buf, name exceptn)
-    add(buf, "]")
+    add(buf, "]\n")
     cstderr.rawWrite buf
     quit 1