Fix stub library definition.

We can use zlib's own version script for this rather than maintaining
our own. I've copied theirs rather than modifying it in place to
avoid local changes to the zlib source and also to allow the
NDK-API-Council check in gerrit to check this file (it only checks
*.map.txt).

This removes a lot of decls that should have been private to zlib,
but it also correctly exposes decls that were added in in JB/JB-MR2,
whereas the old map claimed they were all added in L.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/230
Change-Id: Ib96b41e69d80a3bd28d3b85a0c96e70e6e73fb9d