gcc-wrapper: Use a real date

Analogous to the fake timestamp in the Linux kernel build.

pkgs/build-support/gcc-wrapper/gcc-wrapper.sh

@@ -87,8 +87,8 @@ extraBefore=()
 # When enforcing purity, pretend gcc can't find the current date and
 # time
 if test "$NIX_ENFORCE_PURITY" = "1"; then
-    extraAfter+=('-D__DATE__="??? ?? ????"'
-        '-D__TIME__="??:??:??"'
+    extraAfter+=('-D__DATE__="Jan 01 1970"'
+        '-D__TIME__="00:00:01"'
         -Wno-builtin-macro-redefined)
 fi