Bug 1543595 - Enable linux64-aarch64 builds. r=froydnj

While we don't have an actual need for those builds at the moment, there
is work in progress to get fuzzing builds for aarch64, and as the
previous change showed, the build were busted by other changes since
they were put in place. So we might as well enable them, so as to be
aware of bustage when it happens rather than while working on getting
the fuzzing builds up.

Depends on D27035

Differential Revision: https://phabricator.services.mozilla.com/D27036

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

taskcluster/ci/build/linux.yml

@@ -1127,7 +1127,6 @@ linux64-aarch64/opt:
         secrets: true
         tooltool-downloads: public
         need-xvfb: true
-    run-on-projects: []
     toolchains:
         - linux64-binutils
         - linux64-clang