Fixes 8535 (#8591)

* Goodbye postInitProc

* Give preInitProc its own scope

Avoid any conflict between the variables introduced by preInitProc and
initProc since both are codegen'd in the same function body.

* Fix codegen for global var init in emulated TLS

Fixes #8535

* Add test for #8535

* Keep a bogus stack frame around

* Remove more dead code

tests/threads/t8535.nim

@@ -0,0 +1,16 @@
+discard """
+  output: "0"
+"""
+
+type
+  CircAlloc* [Size: static[int] , T]  =  tuple
+    baseArray           : array[Size,T]
+    index               : uint16
+
+type
+  Job = object of RootObj
+
+var foo {.threadvar.}: CircAlloc[1,Job]
+
+when isMainModule:
+  echo foo.index