[NVPTX] Integrate ptxas to LIT tests

ptxas is a proprietary compiler from Nvidia that can compile PTX to
machine code (SASS). It has a lot of diagnostics to catch errors
in PTX, which can be used to verify PTX output from llc.

Set -DPXTAS_EXECUTABLE=/path/to/ptxas CMake option to enable it.
If this option is not set, then ptxas is substituted to true which
effectively disables all ptxas RUN lines.

LLVM_PTXAS_EXECUTABLE environment variable takes precedence over
the CMake option, and allows to override ptxas executable that is used for LIT
without complete re-configuration.

Differential Revision: https://reviews.llvm.org/D121727