fix compiling with luajit-2.1.0-beta3

It added a function from lua > 5.1, without bumping the language version. The changelog seems safe and Deckard still passes.

Upstream released the bump yesterday. Tested with 2.0.4, 2.1.0-beta{2,3}.

merged

mentioned in commit cecaac0d

Interesting changelog points:

• LJ_GC64: Add JIT compiler support.
• x64/LJ_GC64: Add JIT compiler backend.
• ARM64: Add JIT compiler backend.
• FFI: Compile bitfield loads/stores.