Fix buffer alignment for iOS15/16, macOS12/13 (#1698)

Summary:
Fix buffer alignment for iOS15/16, macOS12/13

Summary of changes:
- Fix `newBufferWithBytesNoCopy` alignment requirements from iOS15/16 and macOS12/13
- Preallocate a list of buffers to use during consecutive inference runs
- Fix missing selectors on older OS versions

Pull Request resolved: https://github.com/pytorch/executorch/pull/1698

Reviewed By: shoumikhin

Differential Revision: D53092160

Pulled By: cccclai

fbshipit-source-id: 284615822d9ab1f150d5f751a344adfebc205c85