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

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]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125052 91177308-0d34-0410-b5e6-96231b3b80d8