Support for llvm::orc2::LLJit

[JCBurnside]

Is your feature request related to a problem? Please describe.
The current ExecutionEngine is using an older engine that has issues with global mappings.

Describe the solution you'd like
I would like a jit engine using LLJIT. the needed c-api functions are anything in llvm_sys::orc2::lljit

Describe possible drawbacks to your solution
might be more difficult to call functions in an idiomatic way.

Describe alternatives you've considered
using the old system and using llvm_sys directly.

Additional context
small sample of how to crate an LLJIT instances https://gist.github.com/JCBurnside/e93d6b95d1caa3746b7fe72073094cc2
sample of using LLJIT (line 95 is the part I am having issues translating) https://github.com/llvm/llvm-project/blob/main/llvm/examples/HowToUseLLJIT/HowToUseLLJIT.cpp

[JCBurnside]

turns out it's just a transmute to get the returned address into a usable function. also updated the gist with a fix for a double free. LLVMOrcCreateLLJIT takes ownership of the builder.