Type-check around find_cc_toolchain result usage

- To handle projects with slightly older @rules_cc gracefully, check
  whether the return from find_cc_toolchain is ToolchainInfo or
  CcToolchainInfo, and pass along the correct variant to the rest of the
  rule logic.

Change-Id: I5d5d8f9bf662e7b407354f86e740c98a00ecf338

build_tools/bazel/kelvin.bzl

diff --git a/build_tools/bazel/kelvin.bzl b/build_tools/bazel/kelvin.bzl
index 7a125c8..4100d15 100644
--- a/build_tools/bazel/kelvin.bzl
+++ b/build_tools/bazel/kelvin.bzl
@@ -72,6 +72,8 @@
         containing the output ELF and BIN.
     """
     cc_toolchain = find_cc_toolchain(ctx)
+    if type(cc_toolchain) != 'CcToolchainInfo':
+        cc_toolchain = cc_toolchain.cc
     feature_configuration = cc_common.configure_features(
         ctx = ctx,
         cc_toolchain = cc_toolchain,