[PATCH] [arm] Implement ARM .arch directive.

[Logan Chien]

  Hi Renato,

  Thanks for your comment.  I was using the top-of-tree binutils when I was developing this patch.  As the result, the behavior of `llvm-mc` should match with the latest `arm-linux-gnueabi-as` (or `arm-linux-gnueabihf-as`) in most cases.  There is only one exception: the default Tag_CPU_name for iwmmxt/iwmmxt2 will be capitalized.

  After reading your comments, I tried to compare the result with binutils 4.6 as well.  I found that binutils 4.6 will emit (Tag_DIV_use, Not_Allowed) for many architectures, including armv7-a.  However, I feel that this behavior is very confusing.  IMO, we should not emit Tag_DIV_use.

  These are the differences as far as I can tell.  Do you have further comments?  Thanks.

  Logan

http://llvm-reviews.chandlerc.com/D2133