tests: disable libomp plan and test in s390x

tests/build-gating-libomp.fmf

@@ -6,10 +6,16 @@ adjust:
       and trigger != commit
       and trigger != build
     enabled: false
+
   - because: "When testing SCL-ized LLVM, the collection must be enabled first"
     environment+:
       WITH_SCL: "scl enable llvm-toolset-13.0"
     when: "collection == llvm-toolset-13.0"
+
+  - because: "libomp not supported in s390x"
+    when: arch == s390x
+    enabled: false
+
 discover:
     how: fmf
     test: libomp

tests/libomp/main.fmf

@@ -9,6 +9,10 @@ tag:
   - not-in-default
 test: "$WITH_SCL ./test.sh"
 adjust:
+  - because: "libomp is not supported in s390x"
+    when: arch == s390x
+    enabled: false
+
   # Common requirements when LLVM is not SCL-ized
   - require:
       - clang