[llvm] [SystemZ] Rename SystemZ ATT Asm dialect to GNU Asm dialect (PR #112800)

[Abhina Sree via llvm-commits]

abhina-sree wrote:

The ATT assembler dialect on SystemZ seems to have been taken from the existing ATT/Intel code. However, on SystemZ, ATT does not hold any meaning. In reality, we are splitting the difference between GNU Asm syntax and HLASM Asm syntax, so it makes sense to rename ATT to GNU instead.

https://github.com/llvm/llvm-project/pull/112800