Improve SM build script detection of LLVM-objdump

Some distributions have an incompatible 'objdump' program, so we should prefer llvm first. Further, 'command' appears to not support multiple targets on some distros.

Parametrize the script a little more.

Reported by: Imarok

Reviewed by: Imarok

Refs #5861

Differential Revision: https://code.wildfiregames.com/D3192