simple multi-gpu configuration

[borg323]

This adds a --gpus=x (0<=x<=8) to specify the number of gpus to use, which is ignored if --backend-opts is used. Calling meson build with -Dgpus_opt=false disables this option, and this is done for the blas and android appveyor builds.

Using --gpus=1 (default) is equivalent to --backend-opts=gpu=0, which is mainly affecting the onednn backend, making the gpu first choice, so I added a fallback to cpu there, if the gpu is not available.

The old behavior is obtained by using --gpus=0, In practice this only makes sense with the onednn backend.

For a higher value of gpus, a backend options line of the form (backend=X,gpu=0),(backend=X,gpu=1) and so on is constructed, with X being either the backend specified with --backend (unless it is multiplexing, demux or roundrobin) or the default backend. The multiplexing backend is used, unless demux or roundrobin is specified with --backend.

[borg323]

I'm having second thoughts about the (ab)use of backend options, made it draft for now.