Add _libm_dummy.c to lib32_libmoldname_a_SOURCES and lib64_libmoldname_a_SOURCES so the archive isn't empty.

This is necessary to support tools that can't process empty archives (eg flexlink on cygwin).

Patch by David Wohlferd limegreensocks@yahoo.com

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@5952 4407c894-4637-0410-b4f5-ada5f102cad1