jit: Supply LLVMGlobalGetValueType() for LLVM < 8.

jit: Supply LLVMGlobalGetValueType() for LLVM < 8.

Commit 37d5babb used this C API function while adding support for LLVM
16 and opaque pointers, but it's not available in LLVM 7 and older.
Provide it in our own llvmjit_wrap.cpp.  It just calls a C++ function
that pre-dates LLVM 3.9, our minimum target.

Back-patch to 12, like 37d5babb.

Discussion: https://postgr.es/m/CA%2BhUKGKnLnJnWrkr%3D4mSGhE5FuTK55FY15uULR7%3Dzzc%3DwX4Nqw%40mail.gmail.com