Bug 1789545 - Add shebang to osx-cross-linker. r=firefox-build-system-reviewers,nalexander

Differential Revision: https://phabricator.services.mozilla.com/D156630

taskcluster/scripts/misc/osx-cross-linker

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 exec $MOZ_FETCHES_DIR/clang/bin/clang -v \
   -fuse-ld=$MOZ_FETCHES_DIR/cctools/bin/$TARGET-ld \
   -mmacosx-version-min=${MACOSX_DEPLOYMENT_TARGET:-10.12} \