main/luajit: use active fork, enable s390x
Let's start using Siddhesh Poyarekar's tree where development is active and there are more fixes and features.
Note that with this commit, we start using snapshots of master branch instead of v2.1 branch. See README for more information.
Now upstream is at https://github.com/siddhesh/LuaJIT.
Also drop the ppc64le patch since it has been reworked and rebased.