Fix AOT on osx+amd64.
Modify the way calls to icalls are made from the icall wrappers. Previously, the address to call was encoded in an OP_PCONST, and various pieces of code looked up the value in the jit icall table to see if it was an icall address. With the new approach, the fact that this is an icall address is explicit through the JIT.
Make the JIT icall wrappers for opcode emulation jit icalls call the C icall function directly on MT.