Add util to check executables/shared libs for missing shared libs for macOS -> no, support on Linux only

[climbfuji]

Is your feature request related to a problem? Please describe.
PR #890 adds a utility to check executables/shared libs for missing shared libraries for Linux. The macOS part still needs to be done post spack-stack 1.6.0

[climbfuji]

@AlexanderRichert-NOAA I suggest we don't try to implement this for macOS. There's no clean and easy way. You can either have full paths to libraries or "rpath-ed" libraries, but the output of otool is not going to tell you if a shared library can be found or not (unlike Linux). You would have to spend quite some time on the ldd_check.py utility to

• check for each linked library with an absolute path if it exists
• collect all rpaths stored in the dylib library
• then loop over all rpaths for each library that is linked as @rpath/mylibrary.dylab and check if it can be found in any of these locations

Given that I haven't used the utility at all, I feel like this is too much effort. I will update the tool and the documentation to make clear this only works on Linux, and send it your way to review if that's ok.

[climbfuji]

See commit 815cd37

[AlexanderRichert-NOAA]

That sounds good. The precipitating issue was one that involved a missing dependency in a recipe (which will apply regardless of platform), plus arguably it's mostly important to catch these things in our tier-1 platform installations, so I have no objection to making it Linux only.

[climbfuji]

Thanks very much!