[extract_symbols.py] Adjust usage of nm again

The previous change to extract_symbols.py means that building on macOS
with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS fails as we try to export some
local symbols, as the regex used to match external symbols wasn't good
enough. Solve this by using the -g option to nm, so we only get
external symbols and don't have to check for local symbols at all.