ARM: permit bare dmb/dsb/isb aliases on Cortex-M0

Cortex-M0 supports these 32-bit instructions despite being Thumb1 only
(mostly). We knew about that but not that the aliases without the default "sy"
operand were also permitted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194094 91177308-0d34-0410-b5e6-96231b3b80d8