[AArch64] Fix data race on RegisterBank initialization.

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

This issue was identified through thread sanitizer.

Reviewers: efriedma, apazos, qcolombet, dsanders

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73587