Add workaround for LuaJIT not supporting >47bit pointers

[daurnimator]

Cqueues mostly uses lightuserdata as unique keys.
It's hacky, but the best workaround seems to be just masking off the top bits.

Solves #223

[daurnimator]

Annoyingly, this makes building with LuaJIT different to building with plain Lua 5.1.
I wonder if distros would want to compile with CQS_USE_47BIT_LIGHTUSERDATA_HACK so that 5.1 and LuaJIT can share the library....

[vcunat]

__arch64__ is a typo, most likely. EDIT: I replaced it with __aarch64__ for now, but I know little about these defines.

[vcunat]

With these two changes I mentioned, it loads and passes some simple tests.

[daurnimator]

__arch64__ is a typo, most likely. EDIT: I replaced it with __aarch64__ for now, but I know little about these defines.

__arch64__ was intentional, it should be defined on 64 bit targets.

[vcunat]

OK, I don't know – for now I mainly cared for the define to get triggered.

I wanted to do some more testing, but I ran into the same problem in luaossl :-/

[daurnimator]

@vcunat wahern/luaossl#173