LLVM_PROFILER_RT_LIB may be set to an absolute path (e.g., in Fedora builds),
but `-l` expects a library name, not a path. After #138273, this caused builds
to fail with a "could not find native static library" error.
This patch updates the build script to split the path into directory and
filename, using `cargo::rustc-link-search` for the directory and
`cargo::rustc-link-lib=+verbatim` for the file. This allows profiler_builtins to
correctly link the static library even when an absolute path is provided.
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