Revert "Remove arm64 from allow_failures"

This reverts commit 212f4d49ba.

It worked on PR, but master branch builds have another build issue.
https://travis-ci.org/ruby/ruby/jobs/608303393

.travis.yml

@@ -378,7 +378,6 @@ matrix:
   include:
     # Build every commit:
     - <<: *x86_64-linux
-    - <<: *arm64-linux
     - <<: *i686-linux
     - <<: *pedanticism
     - <<: *assertions
@@ -386,6 +385,7 @@ matrix:
     - <<: *rubyspec
     - <<: *dependency
     # Build every commit (Allowed Failures):
+    - <<: *arm64-linux
     - <<: *ASAN
     - <<: *MSAN
     - <<: *UBSAN
@@ -400,6 +400,7 @@ matrix:
     - <<: *CALL_THREADED_CODE
     - <<: *NO_THREADED_CODE
   allow_failures:
+    - name: arm64-linux
     - name: -fsanitize=address
     - name: -fsanitize=memory
     - name: -fsanitize=undefined