metal : add BF16 support (#8439)

* ggml : add initial BF16 support

ggml-ci

* metal : add mul_mat_id BF16 support

ggml-ci

* metal : check for bfloat support on the Metal device

ggml-ci

* metal : better var names [no ci]

* metal : do not build bfloat kernels when not supported

ggml-ci

* metal : try to fix BF16 support check

ggml-ci

* metal : this should correctly check bfloat support