openjdk: Fix a cross problem

38eea804e6c02542085c9f232cdd699d96c95a23 dropped the C and C++ compiler prefixes. Probably more work is needed to make cross work, but this at least helps preserve/establish the pattern.

pkgs/development/compilers/openjdk/8.nix

···
     # The configure script was confused by our passing these with full paths,
     # so we explicitly override them to short variants.
     + ''
-      CC=cc
-      CXX=c++
+      CC=${stdenv.cc.targetPrefix}cc
+      CXX=${stdenv.cc.targetPrefix}c++
     '';
 
     configureFlags = [