GPU: give Metal a column in the tuning parameter table

Without one, GPUDefParametersDefaultsDevice.h falls through to
'#error GPU TYPE NOT SET' for GPUCA_GPUTYPE_METAL, which the .metal source
defines, so nothing under Definitions/ could be compiled for the backend at all.

The values are copied from OPENCL, the other portable backend with no
vendor-specific tuning, so Metal starts at WARP_SIZE 32 and
THREAD_COUNT_DEFAULT 256. Real numbers want measuring on a device once the
kernels run. The column is appended last, as the earlier ones were; the order
is cosmetic, since the generator keys off the architecture name.

detect_gpu_arch reports METAL alongside the others so the generator emits the
block. Every existing architecture comes out byte-identical; the generated
device header gains only the Metal block and the architecture comment.

Author: ktf