[ARM] Fix data race on RegisterBank initialization.

Summary:
The initialization of RegisterBank needs to be done only once. The
logic of AlreadyInit has data race, use llvm::call_once instead.

This is continuing work of D73587.

Reviewers: arsenm, rovka, dsanders, t.p.northover, efriedma, apazos

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73605