fix: use `vm_allocate` to allocate CPU backend buffer on macOS (llama/9875)

fix: use `vm_allocate` to allocate CPU backend buffer on macOS (llama/9875)

* fix: use `vm_allocate` to allocate CPU backend buffer on macOS

* fix: switch to `posix_memalign` to keep existing `free()` usages work

* feat: move `GGML_ALIGNED_MALLOC` to `ggml-backend-impl.h`, add support for `vm_allocate` on macOS

* style: formatting

* fix: move const outside of `#ifndef`

* style: formatting

* fix: unused var

* fix: transform `GGML_ALIGNED_MALLOC` and `GGML_ALIGNED_FREE` into functions and add them to `ggml-impl.h`

* fix: unused var

* fix: page align to `GGUF_DEFAULT_ALIGNMENT`

* fix: page align to `TENSOR_ALIGNMENT`

* fix: convert `TENSOR_ALIGNMENT` to a macro

* fix: increase page size to `32` on iOS

* fix: iOS page size

* fix: `hbw_posix_memalign` alignment