Create kernel objects with `@metal launch=false ...` following AMDGPU extension

[PhilipFackler]

For example:
https://github.com/JuliaGPU/Metal.jl/blob/10804a9f7e8a6c49841ed94cc08f8467a2c2b524/src/indexing.jl#L47-L50