build: use .dynsym for selecting symbols to export

Nm uses .symtab by default which is less reliable for this use case.

Previously this would fail if user enabled stripping or used linker
that doesn't add undefined symbols to .symtab, like here:
https://github.com/davidlattimore/wild/issues/819#issuecomment-3215079862

tools/create-exports-NetworkManager.sh

@@ -19,7 +19,7 @@ _sort() {
 
 call_nm() {
     if [ -n "$from_meson" ]; then
-        "${NM:-nm}" "$1" |
+        "${NM:-nm}" -D "$1" |
             sed -n 's/.* \([^ ]\) \([^ ]*\)$/\1 \2/p'
     else
         libtool=(${LIBTOOL:-libtool})
@@ -105,4 +105,3 @@ case "$1" in
         do_generate
         ;;
 esac
-