Recreate usrmerge links in the x64 sysroot.

.gitea/scripts/install-linux-build-dependencies.sh

@@ -80,6 +80,12 @@ case "$target" in
             sudo dpkg-deb --extract "$package" "$sysroot"
         done
 
+        # Recreate Ubuntu's merged-/usr root links. libc development linker
+        # scripts refer to /lib paths, while libc6 stores the files in
+        # /usr/lib; --sysroot expects the same links as a complete rootfs.
+        sudo ln -s usr/lib "$sysroot/lib"
+        sudo ln -s usr/lib64 "$sysroot/lib64"
+
         # Architecture-independent development packages may already be
         # installed on the host, so apt does not download them again. Their
         # pkg-config files contain no host binaries and are safe to reuse with