[mac] CPU dimensions are too precise and miss new Macminis in the pool.

- Drop the following dimensions:
   reserved_for_xcode_version:*
   gpu:none

- Loosen the cpu dimension.
- Restrict to cores:12 to avoid using the VMs.

The failing iPhone6 bots are failing on the main waterfall today
so they are WAI.

See also: https://bugs.chromium.org/p/chromium/issues/detail?id=1257420

Change-Id: I9f0eef7c6904e652a011937ab99c465e9d483172
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/463141
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
2 files changed