Bug 1625884 - add target `linux64-cctools-clang-10-port`. r=nalexander

For building `macosx64-clang-tidy` with `linux64-clang-10` we need to build `cctools`
with `clang-10`.

Differential Revision: https://phabricator.services.mozilla.com/D70064

--HG--
extra : moz-landing-system : lando

taskcluster/ci/toolchain/cctools-port.yml

@@ -23,3 +23,15 @@ linux64-cctools-port:
         toolchain:
             - linux64-clang-9
             - linux64-binutils
+
+linux64-cctools-clang-10-port:
+    treeherder:
+        symbol: TL(cctools-clang-10)
+    run:
+        script: build-cctools-port.sh
+        toolchain-artifact: public/build/cctools.tar.xz
+        tooltool-downloads: internal
+    fetches:
+        toolchain:
+            - linux64-clang-10
+            - linux64-binutils