attempt to fix bootstrapping; refs #3139

compiler/condsyms.nim

@@ -90,3 +90,4 @@ proc initDefines*() =
   defineSymbol("nimnode")
   defineSymbol("nimnomagic64")
   defineSymbol("nimvarargstyped")
+  defineSymbol("nimtypedescfixed")

lib/system.nim

@@ -572,12 +572,14 @@ proc unsafeNew*[T](a: var ref T, size: Natural) {.magic: "New", noSideEffect.}
   ## purposes when you know what you're doing!
 
 proc sizeof*[T](x: T): int {.magic: "SizeOf", noSideEffect.}
-proc sizeof*(x: typedesc): int {.magic: "SizeOf", noSideEffect.}
   ## returns the size of ``x`` in bytes. Since this is a low-level proc,
   ## its usage is discouraged - using ``new`` for the most cases suffices
   ## that one never needs to know ``x``'s size. As a special semantic rule,
   ## ``x`` may also be a type identifier (``sizeof(int)`` is valid).
 
+when defined(nimtypedescfixed):
+  proc sizeof*(x: typedesc): int {.magic: "SizeOf", noSideEffect.}
+
 proc `<`*[T](x: Ordinal[T]): T {.magic: "UnaryLt", noSideEffect.}
   ## unary ``<`` that can be used for nice looking excluding ranges:
   ##