[PATCH] Implement the ldr-pseudo opcode for ARM assembly

[Tim Northover]

> Still waiting for internal approval for phabricator. Would it be ok to
> conduct the review the old-fashioned way for now?

I think it looks reasonable too. I've played around with it and I
can't see anything wrong with the Darwin output.

One small thing I (well, clang) noticed is that the variable
ConstantPoolCounter is unused.

Cheers.

Tim.