ROCm 7.0 and LLVM 21

[Cordell Bloor <cgmb@slerp.xyz>]

Hi ROCm Team,

I had a brief chat with folks on the Debian LLVM IRC and I heard that 
it's possible that LLVM 20 won't be making its way to unstable. I'll 
look at updating the ROCm stack to be based on LLVM 21 instead. Neither 
is currently available on unstable, but both are available in 
experimental. To do this, I will probably have to update rocm-llvm to 7.0.0.

There are a few nice compiler features already available in LLVM 20, 
including `--offload-compress` and 'experimental' support for generic 
targets (via `-mcode-object-version=6 --offload-arch=gfx-11-generic`). 
However, the latter is marked as experimental. As far as I can tell, the 
generic target support is complete in LLVM 20 but the experimental 
warning exists because runtime support was not available at the time. It 
works fine as far as I can tell. With that said, I would suggest only 
adopting `--offload-compress` with LLVM 20. The adoption of generic 
targets can begin with LLVM 21.

Sincerely,
Cory Bloor