Avoid using `fork()` when probing system `libstdc++` (#60254)
Somewhat of a companion to
https://github.com/JuliaLang/julia/pull/60248.
For a small application that has just started up `fork()` is not a huge
concern, but it's quite heavy-handed for Julia- as-a-library scenarios
where resident memory may already be large. Many soft-embedded targets
also do not support fork() well, so it is good for our compatibility to
adjust this.
Rather than relying on the linker to do all of the heavy lifting, this
changes our `libstdcxx` probe sequence to directly parse the
`ld.so.cache` and `libstdc++.so.6` files. As long as we can expect
`/etc/ld.so.cache` to be the same path on all Linux systems, this seems
to be a reliable way to locate system libraries.
