Fix large size increase (around 66%) of our Windows binaries after toolchain upgrade for Firefox 78 ESR

--enable-strip is not available anymore for Windows after https://bugzilla.mozilla.org/show_bug.cgi?id=1639815 landed but it seems this causes a large size increase in our .exe files (around 66%).

Filed https://bugzilla.mozilla.org/show_bug.cgi?id=1651680 to track that on Mozilla's end.