Use moonjit 2.1.2 as base tarball
The LuaJIT project has not been doing releases for a few years, so use
releases from the v2.1 branch of moonjit as the base tarball.
The version number follows moonjit's numbering. This is a risk if
upstream LuaJIT suddenly starts doing releases, but it is not serious
enough since the moonjit v2.1 branch remains in sync. We can mitigate
the version number inconsistency by slowing down moonjit version
I have also dropped changes that are unlikely to make it into LuaJIT.
I will propose a separate package for moonjit v2.2 and beyond since
that will have a very different trajectory from LuaJIT.