Embed English versions of .ds str-Rv-* and .ds str-Ex-* in groff_mdoc.7,
in order to force .Rv and .Ex to output English text.

This is required for the .Rv and .Ex examples in groff_mdoc.7, since it
explains original groff_mdoc.7 bahavior (English text output).
Without this workaround, .Rv and .Ex output Japanese text, since
-dlang=ja_JP.eucJP selected Japanese versions of str-Rv-* and str-Ex-*.