VM: support importc var, ptr/pointer types, cast int <=> ptr/pointer (#12877)

* VM: allow certain hardcoded special var variables at CT
* VM: allow all importc var, cast[int](ptr)
* fix tests tests/vm/tstaticprintseq.nim, tests/cpp/t8241.nim
* VM: == works for ptr/pointer nodes
* bugfix: ==, cast now also works for pointer, not just ptr
* VM supports cast PtrLikeKinds <=> PtrLikeKinds / int
* improve cname handling
* fixup + bug fix
* VM: support cast from ref to int
* address comment: opcLdGlobalDeref => opcLdGlobalDerefFFI
* defensive check against typ == nil

compiler/pragmas.nim

@@ -129,6 +129,8 @@ proc setExternName(c: PContext; s: PSym, extname: string, info: TLineInfo) =
       s.loc.r = rope(extname % s.name.s)
     except ValueError:
       localError(c.config, info, "invalid extern name: '" & extname & "'. (Forgot to escape '$'?)")
+  when hasFFI:
+    s.cname = $s.loc.r
   if c.config.cmd == cmdPretty and '$' notin extname:
     # note that '{.importc.}' is transformed into '{.importc: "$1".}'
     s.loc.flags.incl(lfFullExternalName)