Fix libc detection on various platforms

[ItsHarper]

What's the problem this PR addresses?

Two problems:

1. Improve libc presence detection #6170 broke libc detection on at least Alpine Linux and Chimera Linux (and probably on all other musl-based distributions as well) by looking for the generic string libc in /usr/bin/ldd. On both Chimera and Alpine, ldd --version prints musl libc among the output.

2. getLibc() was doing unnecessary work on OSes other than Windows, Mac, and Linux (I originally thought it would have been crashing, but that's incorrect). It strongly appears to me that getLibc() would throw an error on any OS other than Windows, Mac, and Linux, as the other OSes supported by node have their own libc implementations that are neither glibc nor musl.
   ...

How did you fix it?

1. The strings GNU libc and GNU C Library are searched for instead of just libc.

   • By default, ldd contains the text GNU libc in its --version output, but that can be easily customized at build time using a build configuration option (see here and here). As a fallback, the text GNU C Library is found in the header of the ldd script, and it will probably remain there as long as that file remains a script (as opposed to a compiled binary).

2. I made getLibc() immediately return null on all non-Linux platforms (not just Windows and macOS).

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Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[ItsHarper]

As a side note, while it took a year for anyone to notice that musl support was broken, you'd only notice that it was broken when you tried to install a native package that published musl binaries separately (some of them combine them in a single package).

On the other hand, support for the Unix platforms (not counting Linux and macOS) has almost certainly been broken for 3.5 years (since #3981), and I'm pretty sure yarn was fully crashing on those systems, regardless of which packages you're installing.

Disregard, that last part, I misunderstood how that mapAndFind() code worked.