metal : use `vm_allocate` instead of `posix_memalign` on macOS (#7078)
* fix: use `malloc` instead of `posix_memalign` in `ggml-metal.m` to make it not crash Electron proccesses
* fix: typo
* fix: use `vm_allocate` instead of `posix_memalign`
* fix: don't call `newBufferWithBytesNoCopy` with `NULL` when `ggml_metal_host_malloc` returns `NULL`
* fix: use `vm_allocate` only on macOS