[clang-tidy] Rename objc-avoid-spinlock check to darwin-avoid-spinlock

Summary:
OSSpinLock* are Apple/Darwin functions, but were previously located with ObjC checks as those were most closely tied to Apple platforms before.

Now that there's a specific Darwin module, relocating the check there.

This change was prepared by running rename_check.py.

Contributed By: mwyman

Reviewers: stephanemoore, dmaclach

Reviewed By: stephanemoore

Subscribers: Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang, #llvm

Differential Revision: https://reviews.llvm.org/D68148

llvm-svn: 373392