[X86] Parse and ignore .arch directives

We parse .arch so that some `.arch i386; .code32` code can assemble. It seems
that X86AsmParser does not do a good job tracking what features are needed to
assemble instructions. GNU as's x86 port supports a very wide range of .arch
operands. Ignore the operand for now.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D84900