[PATCH for-4.17] CI: Force CONFIG_XEN_IBT in the buster-gcc-ibt test

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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: 5d59421815d5 ("x86: Use control flow typechecking where possible")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Doug Goldstein <cardoe@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Henry Wang <Henry.Wang@xxxxxxx>

For 4.17: This is bugfix to CI only, to avoid it producing a false negative.
Currently, the test intermittently fails to spot the error it was intended to
identify.  It is very low risk as far as the 4.17 release goes.

https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/3084774561#L373 for
proof that CONFIG_XEN_IBT=y is being fed into allrandom.config
---
 automation/gitlab-ci/build.yaml | 2 ++
 automation/scripts/build        | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/automation/gitlab-ci/build.yaml b/automation/gitlab-ci/build.yaml
index 720ce6e07ba0..65e06c858ef3 100644
--- a/automation/gitlab-ci/build.yaml
+++ b/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
diff --git a/automation/scripts/build b/automation/scripts/build
index 2f15ab3198e6..2d9dd86df904 100755
--- a/automation/scripts/build
+++ b/automation/scripts/build
@@ -12,6 +12,11 @@ cc-ver()
 
 # random config or default config
 if [[ "${RANDCONFIG}" == "y" ]]; then
+
+    # Append job-specific fixed configuration
+    [[ -n "${EXTRA_FIXED_RANDCONFIG}" ]] &&
+        echo "${EXTRA_FIXED_RANDCONFIG}" >> xen/tools/kconfig/allrandom.config
+
     make -j$(nproc) -C xen KCONFIG_ALLCONFIG=tools/kconfig/allrandom.config 
randconfig
     hypervisor_only="y"
 else
-- 
2.11.0