Introduce a PreMain proc in the C codegen

The rationale here is that it has become too hard to step into a program
when #line directives are enabled. You have to skip over many lines of init
code that doesn't have corresponding lines in the nimrod program.
Now, you can just step-out of PreMain and go straight to the useful code
in NimMain.

compiler/cgendata.nim

@@ -114,7 +114,8 @@ type
     injectStmt*: PRope
 
 var
-  mainModProcs*, mainModInit*, mainDatInit*: PRope # parts of the main module
+  mainModProcs*, mainModInit*, otherModsInit*, mainDatInit*: PRope
+    # varuious parts of the main module
   gMapping*: PRope             # the generated mapping file (if requested)
   gModules*: seq[BModule] = @[] # list of all compiled modules
   gForwardedProcsCounter*: int = 0