Add support for LuaJIT's GC64 mode

[agentzh]

LuaJIT has recently added support for the new GC64 mode which allows the full low 47-bit address space to be used by LuaJIT's GC-managed memory.

It seems that this module requires some changes regarding light userdata to actually support this GC64 mode:

http://www.freelists.org/post/luajit/fix-lightud-type-for-48bit-virtual-address

LuaJIT/LuaJIT#230 (comment)

[alyoshark]

Appreciate the effort! Would love to know roughly which version is going to include the fix. Thank you.

[spacewander]

Is there a plan to support GC64 mode? Can't wait to have the ability to use more memory.

[agentzh]

@spacewander Patches welcome.

[spacewander]

Let me repost the discussion in elsewhere:

1. With LuaJIT in GC64 mode, all test cases pass in x64 platform.
2. However, there is a problem with lightuserdata in ARM64 and simillary platform. See the detail in Lightuserdata on ARM64 #1152.
3. GC64 mode slightly drops performance for CPU-bound task.

[agentzh]

We already have #1152 for ARM64 + GC64. We can close this.

[agentzh]

Noted one segmentation fault when running lua-resty-core's test suite with LuaJIT GC64:

openresty/lua-resty-core#144

[agentzh]

@pavelnemirovsky Please do not cross post. It's annoying. Thanks for your cooperation.