[Orc] Directly emit machine code for the x86 resolver block and trampolines.

Bypassing LLVM for this has a number of benefits:

1) Laziness support becomes asm-syntax agnostic (previously lazy jitting didn't
   work on Windows as the resolver block was in Darwin asm).

2) For cross-process JITs, it allows resolver blocks and trampolines to be
   emitted directly in the target process, reducing cross process traffic.

3) It should be marginally faster.

llvm-svn: 251933