--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/assert/tassert_c.nim

@@ -8,7 +8,7 @@ tassert_c.nim(35)        tassert_c
 tassert_c.nim(34)        foo
 assertions.nim(27)       failedAssertImpl
 assertions.nim(20)       raiseAssert
-fatal.nim(39)            sysFatal"""
+fatal.nim(55)            sysFatal"""
 
 proc tmatch(x, p: string): bool =
   var i = 0