Turns out that libgccjit gets installed as a .so file, which the gcc
builder.sh didn't change: It only touched .dylib files; that means
that anything linking in libgccjit.so would receive an "Image not
found" error at load time.
With this change, we invoke `install_name_tool` on .so files too,
adjusting their dynamic linker ID, so that they too can be found.
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