[llvm] [AMDGPU][NFC] Replace gfx940 and gfx941 with gfx942 in llvm/test (PR #125711)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

> > I've got no stake here, but it seems logical to me for dumping tools to still be able to identify obsolete flavours of AMDGPU, so that if somebody needs to inspect one of these objects, they'd be able to see that it is the obsolete version.
> > Specifically, I'm thinking the changes in the Object/AMDGPU/elf-header-flags-mach.yaml and tools/llvm-readobj/ELF/AMDGPU/elf-headers.test tests should be dropped. I'd say the same for the llvm-objdump test, but I expect that's less viable, since it requires llc support to generate the input (could it be switched to yaml2obj instead?).
> 
> Makes sense to me, I'll bring it up for an internal discussion. Thanks for pointing that out, @jh7370!

Per further discussion, we found it best to also drop these targets from the elf tooling.

https://github.com/llvm/llvm-project/pull/125711