libgomp/libgomp.texi: Fix @node order in @menu

While texinfo 7.0.3 does not warn, an older texinfo did complain about:
libgomp.texi:1964: warning: node next `omp_target_memcpy' in menu
`omp_target_memcpy_rect' and in sectioning `omp_target_memcpy_async' differ

libgomp/

	* libgomp.texi (Device Memory Routines): Swap item order to match
	the order of the '@node's of the '@subsection's.