[centos] update to gcc 9

centos/Dockerfile

@@ -8,7 +8,7 @@ LABEL authors="Moritz Maxeiner <moritz.maxeiner@fu-berlin.de>"
 ENV BASH_ENV "/etc/profile"
 RUN yum install -y centos-release-scl && \
     yum install -y epel-release && \
-    yum install -y devtoolset-8 && \
+    yum install -y devtoolset-9 && \
     yum install -y rh-git218-git && \
     yum install -y patch && \
     yum clean all

centos/bash/toolchain.sh

-# GCC 8
-source scl_source enable devtoolset-8
+# GCC 9
+source scl_source enable devtoolset-9
 
 # Git 2.18
 source scl_source enable rh-git218