ci: add a test with the latest gcc and mainline headers

* .gitlab-ci.yml (gcc-x86_64-libdw-mainline): New entry.
* .travis.yml (CC=gcc-8, KHEADERS=torvalds/linux): Likewise.

.gitlab-ci.yml

@@ -8,6 +8,15 @@ variables:
   SLEEP_A_BIT: "sleep 0.2"
   VERBOSE: "1"
 
+gcc-x86_64-libdw-mainline:
+  stage: test
+  script: ci/run-build-and-tests.sh
+  variables:
+    CC: "gcc"
+    TARGET: "x86_64"
+    STACKTRACE: "libdw"
+    KHEADERS: "torvalds/linux"
+
 gcc-x86_64-libdw:
   stage: test
   script: ci/run-build-and-tests.sh

.travis.yml

@@ -24,6 +24,11 @@ matrix:
         - STACKTRACE=libunwind
         - CHECK=coverage
         - KHEADERS=torvalds/linux
+    - compiler: gcc-8
+      env:
+        - TARGET=x86_64
+        - STACKTRACE=libunwind
+        - KHEADERS=torvalds/linux
     - compiler: gcc
       env:
         - TARGET=x86_64