feat(symlinks,execve): support cross-mount symlinks and external binary resolution in execve

- In hook_symlinkat and gkfs_libc, allow storing external target paths when destination is in GekkoFS (e.g. for python venv bin/python3 -> /usr/bin/python3).
- In intercept SYS_execve and SYS_execveat, resolve symlinks targeting external host binaries before passing to kernel execve, keeping argv[0] intact so virtualenvs detect their environment.
- In gkfs_readlink, return external target strings directly without prepending mountdir.
- In get_metadata, stat external symlink targets directly on the host when following links.
- In gkfs_open_impl, delegate opening external symlink targets to host openat.