Provide convenient constructor for LIBXSMM_?GEMM_XARGS also to workaround the issue of using C_NULL_PTR when calling the default c'tor.
Provide convenient constructor for LIBXSMM_?GEMM_XARGS also to workaround the issue of using C_NULL_PTR when calling the default c'tor.

hfp/libxsmm,egeor/libxsmm,hfp/libxsmm,maxhutch/libxsmm,hfp/libxsmm,egeor/libxsmm,hfp/libxsmm,hfp/libxsmm,maxhutch/libxsmm,maxhutch/libxsmm,egeor/libxsmm,egeor/libxsmm,egeor/libxsmm,maxhutch/libxsmm,hfp/libxsmm,egeor/libxsmm