Fix PR18345: ldr= pseudo instruction produces incorrect code when using in inline assembly

This patch fixes the ldr-pseudo implementation to work when used in
inline assembly.  The fix is to move arm assembler constant pools
from the ARMAsmParser class to the ARMTargetStreamer class.

Previously we kept the assembler generated constant pools in the
ARMAsmParser object. This does not work for inline assembly because
a new parser object is created for each blob of inline assembly.
This patch moves the constant pools to the ARMTargetStreamer class
so that the constant pool will remain alive for the entire code
generation process.

An ARMTargetStreamer class is now required for the arm backend.
There was no existing implementation for MachO, only Asm and ELF.
Instead of creating an empty MachO subclass, we decided to make the
ARMTargetStreamer a non-abstract class and provide default
(llvm_unreachable) implementations for the non constant-pool related
methods.

Differential Revision: http://llvm-reviews.chandlerc.com/D2638

llvm-svn: 200777