[ARM] Make ARM::ArchExtKind use 64-bit underlying type, NFCI

Summary:
This patch changes the underlying type of the ARM::ArchExtKind
enumeration to uint64_t and adjusts the related code.

The goal of the patch is to prepare the code base for a new
architecture extension.

Reviewers: simon_tatham, eli.friedman, ostannard, dmgreen

Reviewed By: dmgreen

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D73906