Jenkins: Try building debug builds with optimizations

This should reduce the number of timeouts we see in CI while
still retaining all the useful TCG debug assertions.
Ideally we would use -Og, but that does not work with the
Clang 10 compiler installed on the CI nodes as it is too old.
Using -O1 (which is equivalent to -Og) results in a false-positive
qemu_build_not_reached() with clang10 so we use -O2 for now.

Jenkinsfile

@@ -150,7 +150,7 @@ selectedConfigs.each { config ->
             def extraQemuArgs = ''
             boolean isDebug = config.endsWith('-debug')
             if (isDebug) {
-                extraQemuArgs = '--qemu/configure-options=--enable-rvfi-dii --qemu/build-type=Debug'
+                extraQemuArgs = '"--qemu/configure-options=--enable-rvfi-dii --extra-cflags=-O2 --extra-cxxflags=-O2" --qemu/build-type=Debug'
             }
             def qemuResult = cheribuildProject(target: 'qemu', cpu: 'native', skipArtifacts: true,
                     nodeLabel: null,