OpenMP/nvptx: support 'arch(nvptx64)' as context selector

The main 'arch' context selector for nvptx is, well, 'nvptx';
however, as 'nvptx64' is used as by LLVM, it makes sense
to support it as well.

Note that LLVM has: "The triple architecture can be one of
``nvptx`` (32-bit PTX) or ``nvptx64`` (64-bit PTX)."
GCC effectively only supports the 64bit variant (at least for
offloading). Thus, GCC's 'nvptx' is not quite the same as LLVM's.

The device-compiler part (nvptx_omp_device_kind_arch_isa) uses
TARGET_ABI64 such that nvptx64 is only defined with -m64.

gcc/ChangeLog:

	* config/nvptx/gen-omp-device-properties.sh: Add 'nvptx64' to arch.
	* config/nvptx/nvptx.cc (nvptx_omp_device_kind_arch_isa): Likewise.

libgomp/ChangeLog:

	* libgomp.texi (OpenMP Context Selectors): Add 'nvptx64' as additional
	'arch' value for nvptx.