build: set node_arch to target_cpu in GN

[codebytere]

When building Node.js using the GN build, it would fail when cross-compiling, e.g.

./tools/gn-gen.py out/Release --target_cpu=arm64

becuase node_arch was being improperly set to current_cpu in every case and thus the startup snapshot wouldn't match:

node/src/node_snapshotable.cc

Lines 677 to 684 in eef3030

	if (metadata.node_arch != per_process::metadata.arch) {
	fprintf(stderr,
	"Failed to load the startup snapshot because it was built with"
	"architecture %s and the architecture is %s.\n",
	metadata.node_arch.c_str(),
	NODE_ARCH);
	return false;
	}