MC X86: Accept ".att_syntax prefix" and diagnose noprefix

Fixes PR18916.  I don't think we need to implement support for either
hybrid syntax.  Nobody should write Intel assembly with '%' prefixes on
their registers or AT&T assembly without them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215031 91177308-0d34-0410-b5e6-96231b3b80d8