Enables runtime linking for darwin platforms (#766)

Runtime linking with rpaths works correctly on darwin with only this
change, for dylibs that have the correct extension (dylib) and the
correct rpath configuration. Runtime linking against dynamic libraries
created by bazel targets remains broken, due to them not having install
names set correctly. That is fixed by using
--incompatible_macos_set_install_name after
bazelbuild/bazel#13427.

rust/private/rustc.bzl

@@ -735,7 +735,12 @@ def _compute_rpaths(toolchain, output_dir, dep_info):
     dylibs = [lib for lib in preferreds if lib.basename.endswith(toolchain.dylib_ext) or lib.basename.split(".", 2)[1] == toolchain.dylib_ext[1:]]
     if not dylibs:
         return depset([])
-    if toolchain.os != "linux":
+
+    # For darwin, dylibs compiled by Bazel will fail to be resolved at runtime
+    # without a version of Bazel that includes
+    # https://github.com/bazelbuild/bazel/pull/13427. This is known to not be
+    # included in Bazel 4.1 and below.
+    if toolchain.os != "linux" and toolchain.os != "darwin":
         fail("Runtime linking is not supported on {}, but found {}".format(
             toolchain.os,
             dep_info.transitive_noncrates,