Add lib64 in the llvm-toolset config (#325)

Author: shahmishal

swift-ci/master/centos/7/Dockerfile

@@ -41,6 +41,9 @@ RUN yum install --enablerepo=centosplus  -y \
 
 RUN echo -e ". /opt/rh/sclo-git25/enable\n. /opt/rh/llvm-toolset-7.0/enable\n. /opt/rh/devtoolset-8/enable\n" >> /home/build-user/.bashrc
 
+RUN echo /opt/rh/llvm-toolset-7.0/root/usr/lib64/ > /etc/ld.so.conf.d/llvm-toolset.conf
+RUN ldconfig
+
 RUN sed -i -e 's/\*__block/\*__libc_block/g' /usr/include/unistd.h
 
 USER build-user