perl-5.20: break dependency on the compiler that built it

This is a major closure size reduction on Darwin, and probably a less
significant one on Linux. On darwin, retaining the compiler means adding
clang and its dependency llvm to the perl closure, which gives us ~400MB
of extra stuff. Considering that Nix itself depends on this version of
perl, that makes cutting a new Nix release rather unpleasaont Darwin.

After this patch, I was able to get the `nixUnstable` closure down to
21MB after feeding it into a .tar.xz (123MB before compression). There's
still room for improvement but this should carry us over until we split
outputs.