* gcc: support runtime ability to not install spack rpaths
Fixes#26582 .
* gcc: Fix malformed specs file and add docs
The updated docs point out that the spack-modified GCC does *not*
follow the usual behavior of LD_RUN_PATH!
* gcc: fix bad rpath on macOS
This bug has been around since the beginning of the GCC package file:
the rpath command it generates for macOS writes a single (invalid)
rpath entry.
* gcc: only write rpaths for directories with shared libraries
The original lib64+lib was just a hack for "in case either has one" but
it's easy to tell whether either has libraries that can be directly
referenced.
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