Added the B9.3.19 SUBS PC, LR, #imm (Thumb2) system instruction.

While the .td entry is nice and all, it takes a pretty gross hack in
ARMAsmParser::ParseInstruction() because of handling of other "subs"
instructions to get it to match.  Ran it by Jim Grosbach and he said it was
about what he expected to make this work given the existing code.

rdar://14214063

llvm-svn: 187530