Support PNaCl

[TurkeyMan]

PNaCl is an important target where LuaJIT is not supported.

[Uzume]

I am not sure this means much at this point but PNaCl has been deprecated in favor of WebAssembly since May 30, 2017:

• https://blog.chromium.org/2017/05/goodbye-pnacl-hello-webassembly.html

It is scheduled for complete end of support June 2022:

• https://blog.chromium.org/2020/08/changes-to-chrome-app-support-timeline.html

Since this is based on DynASM from LuaJIT, I doubt support for WebAssembly will come any time soon unless LuaJIT adds support for such a target (which seems unlikely).