[PR libgomp/65099] nvptx mkoffload: pass "-m32" or "-m64" to the compiler

... depending on "-foffload-abi=[...]".

Coding style/code copied from gcc/config/i386/intelmic-mkoffload.c for
consistency.

	gcc/
	* config/nvptx/mkoffload.c (target_ilp32): New variable.
	(main): Set it depending on "-foffload-abi=[...]".
	(compile_native, main): Use it to pass "-m32" or "-m64" to the
	compiler.

From-SVN: r222583