fix: use vm_allocate instead of posix_memalign for Metal on macOS

[giladgd]

Creating a context in an Electron app using node-llama-cpp crashes the process with some models (issue), so I've investigated what's happening and found that allocating a large memory block using posix_memalign is the culprit.

For some reason, it happens only on Electron and not on Nodejs, but I couldn't figure out why.

From my testings in Electron:

• posix_memalign((void **) &data, 16384, 587218944) - works fine
• posix_memalign((void **) &data, 16384, 1073741824) - crashes the process with SIGTRAP

Tested on an M1 Max machine with 32GB of RAM

I tried switching from posix_memalign to malloc in ggml-metal.m, and it seems that everything still works correctly, but maybe I'm missing something.
I assume that posix_memalign is used there for a reason, but since it seems to me that everything still works great with malloc, maybe the original reason for using posix_memalign is irrelevant by now?

I'm not sure whether the change I made in this PR is a good idea, so I opened it so someone more knowledgable in this area can take a look.

This may be a bug specific to Electron, so I shared my findings on the Electron repo, but since I haven't noticed any side effect of this workaround in llama.cpp, I think it may be a good idea to also solve this issue here.

[slaren]

Metal requires a page-aligned pointer, which is why posix_memalign is used.

[giladgd]

I've switched to use vm_allocate instead since I found that this is what the Apple documentation recommends, and it seems to also fix the issue with Electron.

vm_allocate also allocates page-aligned memory.
Is the new change I made ok?

[giladgd]

I've been using this for the past few days, and it seems to work great.
I've seen that all the tests passed, so I think this may be a good solution to the Electron issue without affecting other things.