Add tests for rip-rel/abs resolution with AT&T/Intel syntax.

Also adds x86 tests that check the same absolute addressing.

Add tests for nasm rel/abs addressing

Add and use ARMTargetStreamer from llvm.

Added the missing ARMTargetStreamer from llvm, instantiate it in
ARMAsmParser, use it to implement constant pool.

Refs: #28

arm: propagate INVALIDOPERAND & MISSINGFEATURE from parser to frontend API. this partly handles issue #28