[ARM] Make ARM::ArchExtKind use 64-bit underlying type (part 2), NFCI

Summary:
After following Simon's suggestion about additional testing posted at
https://reviews.llvm.org/D73906, I found several more places that
need to be updated.

Reviewers: simon_tatham, dmgreen, ostannard, eli.friedman

Reviewed By: simon_tatham

Subscribers: merge_guards_bot, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73963