[libc] Set minimum CUDA PTX feature to +ptx60

Summary:
The `+ptx` features correspond to the related CUDA version. We require a
certain set of features from the `ptxas` assembler, which is tied to the
CUDA version. Some of the ones set here were insufficient, so I am
simply setting a cutoff to the CUDA 9.0 release as the minimum. This
roughly corresponds to what should be required for sm_60 to be compiled
with the source.