[llvm] [llvm-lit][NVPTX] Use the shell's `ptxas` if `LLVM_PTXAS_EXECUTABLE` isn't set (PR #132810)

[Artem Belevich via llvm-commits]

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@@ -345,7 +346,7 @@ def version_int(ver):
 
 
 ptxas_executable = (
-    os.environ.get("LLVM_PTXAS_EXECUTABLE", None) or config.ptxas_executable
+    os.environ.get("LLVM_PTXAS_EXECUTABLE", None) or config.ptxas_executable or shutil.which("ptxas")
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Artem-B wrote:

I am not convinced that it is a good idea to enable ptxas when it's found in the PATH, but is not configured by the user.

ptxas tests are optional, because `ptxas` is not something that LLVM build normally requires.
While enabling ptxas tests for those who work on NVPTX would be very useful, it will also enable it for those who happen to have ptxas installed for unrelated reasons but have nothing to do with NVPTX. The main issue is that when something gets enabled exclusively because some executable is present in the PATH, there's no good way to disable those tests.

I'd be fine with a cmake option setting LLVM_PTXAS_EXECUTABLE to the binary found in PATH (or explicitly configures) and propagating it to list test, along with the way to let user disable PTXAS tests (or not enable them) if they need to.

https://github.com/llvm/llvm-project/pull/132810