Fix stack bottom initialization for non-main modules.

Because PreMain() was now called before the new stack bottom
initialization, it still relied on the old version of initStackBottom(),
which may not handle the top few words of the stack correctly. This
patch also sets the stack bottom in PreMain() using the new approach.

compiler/cgen.nim

@@ -949,12 +949,23 @@ proc genFilenames(m: BModule): PRope =
 
 proc genMainProc(m: BModule) =
   const 
+    # The use of a volatile function pointer to call Pre/NimMainInner
+    # prevents inlining of the NimMainInner function and dependent
+    # functions, which might otherwise merge their stack frames.
     PreMainBody =
-      "\tsystemDatInit();$N" &
+      "void PreMainInner() {$N" &
       "\tsystemInit();$N" &
       "$1" &
       "$2" &
-      "$3"
+      "$3" &
+      "}$N$N" &
+      "void PreMain() {$N" &
+      "\tvoid (*volatile inner)();$N" &
+      "\tsystemDatInit();$N" &
+      "\tinner = PreMainInner;$N" &
+      "$4" &
+      "\t(*inner)();$N" &
+      "}$N$N"
 
     MainProcs =
       "\tNimMain();$N"
@@ -962,9 +973,6 @@ proc genMainProc(m: BModule) =
     MainProcsWithResult =
       MainProcs & "\treturn nim_program_result;$N"
 
-    # The use of a volatile function pointer to call NimMainInner
-    # prevents inlining of the NimMainInner function and dependent
-    # functions, which might otherwise merge their stack frames.
     NimMainBody =
       "N_CDECL(void, NimMainInner)(void) {$N" &
         "$1" &
@@ -1047,8 +1055,8 @@ proc genMainProc(m: BModule) =
       platform.targetOS == osStandalone: "".toRope
     else: ropecg(m, "\t#initStackBottomWith((void *)&inner);$N")
   inc(m.labels)
-  appcg(m, m.s[cfsProcs], "void PreMain() {$N" & PreMainBody & "}$N$N", [
-    mainDatInit, gBreakpoints, otherModsInit])
+  appcg(m, m.s[cfsProcs], PreMainBody, [
+    mainDatInit, gBreakpoints, otherModsInit, initStackBottomCall])
 
   appcg(m, m.s[cfsProcs], nimMain, [mainModInit, initStackBottomCall, toRope(m.labels)])
   if optNoMain notin gGlobalOptions: