build: call AC_PATH_TOOL for dsymutil in macOS cross-compile

While testing bitcoin#19530 I noticed that we couldn't call dsymutil after LTO:
```bash
../libtool: line 10643: x86_64-apple-darwin16-dsymutil: command not found
```

This updates configure to call `AC_PATH_TOOL` so that we end up with the
full path to dsymutil, similar to `otool` and `install_name_tool`, ie:
`/bitcoin/depends/x86_64-apple-darwin16/share/../native/bin/x86_64-apple-darwin16-otool`.

configure.ac

@@ -642,6 +642,7 @@ case $host in
            BUILD_OS=darwin
            ;;
          *)
+           AC_PATH_TOOL([DSYMUTIL], [dsymutil], dsymutil)
            AC_PATH_TOOL([INSTALLNAMETOOL], [install_name_tool], install_name_tool)
            AC_PATH_TOOL([OTOOL], [otool], otool)
            AC_PATH_PROGS([GENISOIMAGE], [genisoimage mkisofs],genisoimage)