Remove the MCR asm parser hack and start using the custom target specific asm

parsing of operands introduced in r125030. As a small note, besides using a more
generic approach we can also have more descriptive output when debugging
llvm-mc, example:

mcr  p7, #1, r5, c1, c1, #4

note: parsed instruction:
  ['mcr', <ARMCC::al>,
          <coprocessor number: 7>,
          1,
          <register 73>,
          <coprocessor register: 1>,
          <coprocessor register: 1>,
          4]
llvm-svn: 125052