[X86][AsmParser] Treat '%' as the modulo operator under Intel syntax

It can't be a register prefix, anyway. This is consistent with the masm
docs on MSDN: https://msdn.microsoft.com/en-us/library/t4ax90d2.aspx

This is a straight-forward extension of our support for "MOD"
implemented in https://reviews.llvm.org/D33876 / r306425

llvm-svn: 317011