os/linux/ld: handle nonexistent ld.so.conf more gracefully

[carlocab]

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Fixes #18458

[Bo98]

This might make sense when bootstrapping glibc so agree with this change for that.

As a fix for #18458 though: are we sure we're getting the system directories correctly? Otherwise it'll flag everything as broken linkage.

[carlocab]

Added some defaults based on the output of OS::Linux::Ld.library_paths in a codespace. Also included the location of ld.so.conf as reported at #18458.

[carlocab]

Reverted to original changes per discussion on Zoom.