PyPy >= 3.8 supports a standard prefix installation, where older
versions always used a portable/developent style installation.
If this is a standard prefix installation, skip the logic that symlinks
libraries in a portable PyPy distribution.

Fixes: pypa#2309

1. Portable PyPy3.8 build, as downloaded from upstream.
   This should include the provided shared libraries, libpypy3-c.so has
   an rpath of $ORIGIN/../lib to point to them.
2. Debian's /usr/lib/pypy3 layout for PyPy3 < 3.8.
   This should include libpypy3-c.so.
3. Debian's /usr layout for PyPy3 > 3.8.
   This should exclude libpypy3-c.so, as it's on the standard linker
   search path.
   It should also not include anything else from /usr/lib. (See pypa#2309)

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>