Deprecate metal due to recent MTLCompilerService limitations

[flaub]

See: #937

The metal HAL on recent versions of macOS fail to compile due to new limitations on the number of temporary registers allowed in a given function. There doesn't seem to be any easy/low cost way to fix the code generator for eval_check, so our best option at this time is to deprecate Metal support and hope that:

1. Apple raises the limitation in a future macOS release
2. New zirgen compiler will be able to produce a more structured function that doesn't require so many outstanding registers at any given time.

[flaub]

https://developer.apple.com/forums/thread/712889

[weikengchen]

Since M2 seems to be fine, maybe it is okay not to deprecate metal?

[flaub]

AFAIK, this failure occurs on M1 and M2. It seems to work if you happen to have an older version of the Metal compiler. We have a caching system that attempts to avoid long compile times, but this cache might make it seem that things are working even on newer versions.

[flaub]

Metal seems to be working in latest macOS again.

[weikengchen]

That is good news!