NASM/YASM: Fix object format when CXX is enabled but not C

I worked on a small project that is C++ with one part written in ASM. The ASM part I am compiling with yasm. I am on a 64 bit platform so I would like the output to be 64bit as well. The script that sets the output format flags for yasm does currently not check the C++ compiler format even if the varaible CMAKE_C_SIZEOF_DATA_PTR is not defined at all. This is what I fixed here.

Topic-rename: nasm-with-CXX