tools: fix c++ code coverage on macOS

Recent libtool on macOS does not support the --coverage flag that was
being passed through in the final linking stage.
This change patches gyp-mac-tool to not pass the --coverage flag through
to libtool. It is now possible to generate code coverage for src/ on
macOS.

Fixes: nodejs#19057

tools/gyp/pylib/gyp/xcode_emulation.py

@@ -949,6 +949,8 @@ def GetLibtoolflags(self, configname):
     libtoolflags = []
 
     for libtoolflag in self._Settings().get('OTHER_LDFLAGS', []):
+      if libtoolflag == "--coverage":
+        continue
       libtoolflags.append(libtoolflag)
     # TODO(thakis): ARCHS?