Disable LuaJIT on non-x86 architectures

The version of LuaJIT in port doesn't work well (or at all) for neovim
on non-x86 architectures. Plus, some users (at least the submitter, who
made a good argument for it) may not want LuaJIT at all.

So, make LuaJIT an OPTION. Enable it by default, and exclude it from
all the archs that LuaJIT+neovim doesn't work on. Fall back instead on
normal Lua.

PR: 238079
Submitted by: Greg V
MFH: 2019Q2