ARM: bkpt has an implicit immediate constant 0

The bkpt mnemonic has an implicit immediate constant of 0 unless otherwise
specified.  Add an instruction alias for the unvalued breakpoint mnemonic to
treat it as a 0.  This improves compatibility with GNU AS.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197913 91177308-0d34-0410-b5e6-96231b3b80d8