gitlab-ci: Test building linux-user targets on CentOS 7

gitlab-ci: Test building linux-user targets on CentOS 7

Add a configuration tested by Peter Maydell (see [1] and [2])
but not covered in our CI [3]:

  [705/2910] Compiling C object libqemu-arm-linux-user.fa.p/linux-user_strace.c.o
  FAILED: libqemu-arm-linux-user.fa.p/linux-user_strace.c.o
  ../linux-user/strace.c: In function 'do_print_sockopt':
  ../linux-user/strace.c:2831:14: error: 'IPV6_ADDR_PREFERENCES' undeclared (first use in this function)
           case IPV6_ADDR_PREFERENCES:
                ^

This job currently takes 31 minutes 32 seconds ([4]).

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg05086.html
[2] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg05379.html
[3] https://gitlab.com/philmd/qemu/-/jobs/977408284
[4] https://gitlab.com/philmd/qemu/-/jobs/978223286

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210121172829.1643620-3-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>