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