gitlab-ci: Enable llvmpipe in ARM build jobs

v2:
* Use LLVM 8 from buster-backports
v3:
* Use LLVM 7 again for armhf, llvmpipe is still broken there with LLVM 8

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7cde294..d534345 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -202,14 +202,13 @@
       -D gallium-va=false
       -D gallium-xa=false
       -D gallium-nine=false
-      -D llvm=false
 
 .meson-arm:
   extends: .meson-cross
   image: $DEBIAN_ARM64_IMAGE
   variables:
     VULKAN_DRIVERS: freedreno
-    GALLIUM_DRIVERS: "etnaviv,freedreno,kmsro,lima,nouveau,panfrost,tegra,v3d,vc4"
+    GALLIUM_DRIVERS: "etnaviv,freedreno,kmsro,lima,nouveau,panfrost,swrast,tegra,v3d,vc4"
     EXTRA_OPTION: >
       -D I-love-half-baked-turnips=true
   needs:
@@ -221,6 +220,7 @@
   extends: .meson-arm
   variables:
     CROSS: armhf
+    LLVM_VERSION: "7"
 
 meson-arm64:
   extends: