--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.

compiler/ccgexprs.nim

@@ -2675,10 +2675,13 @@ proc expr(p: BProc, n: PNode, d: var TLoc) =
       line(p, cpsStmts, "(void)(" & a.r & ");\L")
   of nkAsmStmt: genAsmStmt(p, n)
   of nkTryStmt, nkHiddenTryStmt:
-    if p.module.compileToCpp and optNoCppExceptions notin p.config.globalOptions:
+    case p.config.exc
+    of excGoto:
+      genTryGoto(p, n, d)
+    of excCpp:
       genTryCpp(p, n, d)
     else:
-      genTry(p, n, d)
+      genTrySetjmp(p, n, d)
   of nkRaiseStmt: genRaiseStmt(p, n)
   of nkTypeSection:
     # we have to emit the type information for object types here to support