[NVPTX] Fix issues in ptxas integration to LIT tests

1) Fixed a typo in PTXAS_EXECUTABLE CMake variable (PXTAS -> PTXAS).

2) Version check was implemented incorrectly,
   now version (major, minor) is converted to int for comparison.

3) ptxas -arch argument was incorrect (or missing) in 3 tests.

Differential Revision: https://reviews.llvm.org/D127866