CI: Force CONFIG_XEN_IBT in the buster-gcc-ibt test

buster-gcc-ibt is a dedicated test to run a not-yet-upstreamed compiler patch
which is relevant to CONFIG_XEN_IBT in 4.17 and later.

Force it on, rather than having 50% of the jobs not testing what they're
supposed to be testing.

Fixes: 5d59421815 ("x86: Use control flow typechecking where possible")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
[stefano: minor code style improvement]
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>

automation/gitlab-ci/build.yaml

@@ -299,6 +299,8 @@ debian-buster-gcc-ibt:
   variables:
     CONTAINER: debian:buster-gcc-ibt
     RANDCONFIG: y
+    EXTRA_FIXED_RANDCONFIG: |
+      CONFIG_XEN_IBT=y
 
 debian-unstable-clang:
   extends: .clang-x86-64-build

automation/scripts/build

@@ -12,6 +12,12 @@ cc-ver()
 
 # random config or default config
 if [[ "${RANDCONFIG}" == "y" ]]; then
+
+    # Append job-specific fixed configuration
+    if [[ -n "${EXTRA_FIXED_RANDCONFIG}" ]]; then
+        echo "${EXTRA_FIXED_RANDCONFIG}" >> xen/tools/kconfig/allrandom.config
+    fi
+
     make -j$(nproc) -C xen KCONFIG_ALLCONFIG=tools/kconfig/allrandom.config randconfig
     hypervisor_only="y"
 else