do not apply fix_lib64 for PyPy

The current version of virtualenv breaks when trying to create a virtualenv with PyPy on a Gentoo/AMD64 system, where `fix_lib64()` logic would be triggered. Inside the function are two `assert`'s, the first of which not true under PyPy's path scheme.

This commit makes `fix_lib64()` PyPy-aware, disabling the (now inappropriate) fix if the environment would otherwise be subject to `lib64` symlinking.

Note: you can't test this out with Gentoo-packaged PyPy, because of a patch introduced by Gentoo bug [#373487](https://bugs.gentoo.org/show_bug.cgi?id=373487). The distutils patch must be reverted, or you will get `Access denied` error (ordinary user) / overwrite system scripts (root).