[XLA:GPU] Don't crash with --vmodule=gpu_compiler=2 if we can't run ptxas.

At --vmodule=gpu_compiler=2, we run ptxas over our generated PTX, to
validate it, and also to dump out stats like the number of registers
used.

But previously, this would fail if your GPU was anything other than
sm_35 (i.e. K20/40/80), because we didn't pass down cc_major/cc_minor to
ptxas.  And moreover, if ptxas failed to compile your program, we'd
LOG(FATAL), which is probably no what you want.

This change fixes both those issues.  Tested on my local GTX1080.

PiperOrigin-RevId: 172403304