install-deps,rpm: enable devtoolset-8 on aarch64 also

install-deps,rpm: enable devtoolset-8 on aarch64 also

in 5ae3b06e, we left aarch64 with devtoolset-7, but aarch64 builders are
also suffering from http://tracker.ceph.com/issues/38892. so i installed
devtoolset-8 on all aarch64 builders manually from the rpm packages
downloaded from koji builder:

- https://cbs.centos.org/koji/buildinfo?buildID=24923
- https://cbs.centos.org/koji/buildinfo?buildID=24931

libasan5, libubsan1 are not installed.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 00f19923b4225c25480ddb3c09a898a2444fbeb9)