Unexport even more symbols (#13214)

* system/gc: don't export markStackAndRegisters

* compiler/cgen: unexport internal symbols

As these functions are Nim-specific walkaround against C's optimization
schemes, they don't serve any purpose being exported.

* compiler/cgen: don't export global var unless marked

* compiler/ccgthreadvars: don't export threadvar unless marked

* tests/dll/visibility: also check for exports

This ensure that these changes don't break manual exports.

* compiler/cgen: hide all variables created for constants

* compiler/ccgtypes: don't export RTTI variables

* compiler/ccgexprs: make all complex const static

* nimbase.h: fix export for windows

* compiler/cgen, ccgthreadvars: export variables correctly

For C/C++ variables, `extern` means that the variable is defined in an
another unit. Added a new N_LIB_EXPORT_VAR to correctly export
variables.

lib/system/gc.nim

@@ -682,7 +682,7 @@ proc gcMark(gch: var GcHeap, p: pointer) {.inline.} =
   `CLANG_NO_SANITIZE_ADDRESS` in `lib/nimbase.h`.
  ]#
 proc markStackAndRegisters(gch: var GcHeap) {.noinline, cdecl,
-    codegenDecl: "CLANG_NO_SANITIZE_ADDRESS $# $#$#".} =
+    codegenDecl: "CLANG_NO_SANITIZE_ADDRESS N_LIB_PRIVATE $# $#$#".} =
   forEachStackSlot(gch, gcMark)
 
 proc collectZCT(gch: var GcHeap): bool =