[llvm] [MC][DecoderEmitter] Fix build warning: explicit specialization cannot have a storage class (PR #156375)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Note that @MaskRay 's earlier fix https://github.com/llvm/llvm-project/commit/a7d1a652c2e7872939caf22d5209a060f7dccd7e sort of works, because it just changed RISCV and not AMDGPU. Which is a good stopgap and reduces the urgency of this (more correct) fix.

https://github.com/llvm/llvm-project/pull/156375