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

[Rahul Joshi via llvm-commits]

jurahul wrote:

Note, @kazutakahirata added one more fix by making them inline, but I suspect its susceptible to the same issue. Maybe not a linker error, but since we have 2 different specializations for uint64_t with different values (64 vs 48). wrong ones being picked up potentially. In practice likely not right now, but I still feel my fix to avoid any sort of visibility of these specializations across targets through the `llvm::MCD` is a more robust fix.

Apologies for the churn over the long weekend :(

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