Enabling Developer Toolset 7 for Jenkins' lo_tb_master_linux

...aka "Tinderbox on Master for Linux-DbgUtil",
<https://ci.libreoffice.org/job/lo_tb_master_linux_dbg/>.  This is similar to
ab8454eb "Enabling Developer Toolset 7 for
Jenkins' lo_tb_master_linux_dbg".  However, unlike lo_tb_master_linux_dbg, which
is restricted to CentOS-based tb75-lilith, lo_tb_master_linux is restricted to
label TB_Rel, which is currently matched to CentOS-based tb75-lilith,
tb76-maggie, and tb76-pollux, plus non-CentOS based gandalf.  The latter thus
doesn't have Developer Toolset 7, so setting CC/CXX this way won't work for
gandalf.  I'm thus removing gandalf from
<https://ci.libreoffice.org/label/TB_Rel/>.

Change-Id: Ied305476347d107db4087f710c05300c84ae7e85
Reviewed-on: https://gerrit.libreoffice.org/64444
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

distro-configs/Jenkins/Linux_rel_master.conf

 --enable-kde4
+CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
+CXX=/opt/rh/devtoolset-7/root/usr/bin/g++