--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.

tests/exception/tfinally4.nim

@@ -1,5 +1,39 @@
 discard """
-  output: "B1\nA1\n1\nB1\nB2\ncatch\nA1\n1\nB1\nA1\nA2\n2\nB1\nB2\ncatch\nA1\nA2\n0\nB1\nA1\n1\nB1\nB2\nA1\n1\nB1\nA1\nA2\n2\nB1\nB2\nA1\nA2\n3"
+  output: '''
+B1
+A1
+1
+B1
+B2
+catch
+A1
+1
+B1
+A1
+A2
+2
+B1
+B2
+catch
+A1
+A2
+0
+B1
+A1
+1
+B1
+B2
+A1
+1
+B1
+A1
+A2
+2
+B1
+B2
+A1
+A2
+3'''
 """
 
 # More thorough test of return-in-finaly