Make snapshot load position-independent (match libraries by basename)

[dg1sbg]

Problem

snapshot_load re-resolves the C++ libraries a snapshot references via core::library_with_name, which accepted a loaded library only if a recorded library name was a path suffix of the loaded library's path:

name.size() <= libName.size() && name == libName.substr(libName.size() - name.size())

The recorded name is the absolute path captured by dladdr at save time. So if a snapshot's libraries are later loaded from a different directory than at save — a relocated or bundled install, a different install prefix, or a different machine — the suffix test fails and snapshot_load aborts in snapshotSaveLoad.cc with "Unable to find library: …". On aarch64 Linux, a save-lisp-and-die :executable t image SIGABRTs before its banner once it (and its libraries) are moved.

Fix

Match libraries by basename (SONAME) instead of by save-time path suffix. The nm/dlsym load-address computation that follows already uses the matched, currently-loaded path, so the matcher was the only place bound to the save-time path.

This makes save-lisp-and-die executables and snapshot files relocatable across prefixes/machines — useful for redistributing images and for embedded deployment. It is not aarch64-specific; it helps every platform.

Testing

aarch64 Linux (Ubuntu 24.04, LLVM 18, boehmprecise): a :executable snapshot whose libLLVM/libstdc++/libclasp were relocated into a separate bundle directory and run via LD_LIBRARY_PATH now boots from its embedded snapshot and runs (*features* contains :clasp; fib, compile, funcall all work). Same-tree behavior is unchanged (the recorded path's basename equals the loaded basename).

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