Bug 1616758 - Fix the linux64-dump-syms toolchain. r=froydnj

The script was not executable, and apparently it is not happy with
anything other than clang.

Differential Revision: https://phabricator.services.mozilla.com/D63447

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

taskcluster/ci/toolchain/dump-syms.yml

@@ -20,6 +20,8 @@ linux64-dump-syms:
         toolchain-artifact: public/build/dump_syms.tar.xz
     fetches:
         toolchain:
+            - linux64-binutils
+            - linux64-clang
             - linux64-rust
 
 win64-dump-syms:

taskcluster/scripts/misc/build-dump-syms.sh

@@ -7,6 +7,9 @@ PROJECT=dump_syms
 case "$(uname -s)" in
 Linux)
     COMPRESS_EXT=xz
+    export RUSTFLAGS=-Clinker=clang++
+    export CXX=clang++
+    export PATH=$MOZ_FETCHES_DIR/clang/bin:$MOZ_FETCHES_DIR/binutils/bin:$PATH
     ;;
 MINGW*)
     UPLOAD_DIR=$PWD/public/build