Adding os_version to conda-forge.yml

[jakirkham]

As this is a newer feedstock, it was brought up after the work on migrating CTK feedstocks to use {{ stdlib("c") }} occurred: conda-forge/cuda-feedstock#26

While this feedstock does have {{ stdlib("c") }}, it currently doesn't specify os_version in conda-forge.yml. Could we please start by adding some values likes these to conda-forge.yml?

os_version:
  linux_aarch64: cos7

Note: It looks like this skips linux_64 and linux_ppc64le. So have left those out