Fix incorrect tools label for _wrapper_device_link

[cloudhan]

Label, for example, :cuda/bin/crt/link.stub is incorrect in

rules_cuda/cuda/private/repositories.bzl

Lines 69 to 76 in 3ce19bc

	if repository_ctx.path(cuda_path + "/bin/nvlink" + bin_ext).exists:
	nvlink = ":cuda/bin/nvlink{}".format(bin_ext)
	if repository_ctx.path(cuda_path + "/bin/crt/link.stub").exists:
	link_stub = ":cuda/bin/crt/link.stub"
	if repository_ctx.path(cuda_path + "/bin/bin2c" + bin_ext).exists:
	bin2c = ":cuda/bin/bin2c{}".format(bin_ext)
	if repository_ctx.path(cuda_path + "/bin/fatbinary" + bin_ext).exists:
	fatbinary = ":cuda/bin/fatbinary{}".format(bin_ext)

Prior to #66 it was, @local_cuda//:cuda/bin/crt/link.stub which resolves to <local_cuda>/cuda/bin/crt/link.stub , but the generated BUILD file resides in <local_cuda>/toolchain/BUILD, so :cuda/bin/crt/link.stub actually resolves to <local_cuda>/toolchain/cuda/bin/crt/link.stub which does not exists.

[cloudhan]

@jsharpe I think we should have a label_helper for creating Label of @rules_cuda and @local_cuda instead of str, so that bazel can help with the remapping.

Previous try of str(Label("@rules_cuda//cuda/dummy:nvlink")) caused the conversion back to str as //cuda/dummy:nvlink instead of the canonical form @<rules_cuda_remapped_name>//cuda/dummy:nvlink in the rules_cuda repo.