CI: Pass -O2 for annocheck

Because `optflags` is pasted into the invocation line after `CC`, we were
building with -O1 unintentionally. You can see this in the configuration
summary: https://github.com/ruby/ruby/actions/runs/3933391169/jobs/6727044423#step:9:753

The check actually fails with -O2. To make it pass, upstream suggest
that we use the annocheck GCC plugin. Since it requires building from
source as the debian package for it isn't ready yet, punt on it for now
and use `--skip-gaps`.

Co-authored-by: Jun Aruga <jaruga@redhat.com>

.github/workflows/compilers.yml

@@ -83,12 +83,13 @@ jobs:
             container: gcc-11
             env:
               # Minimal flags to pass the check.
-              default_cc: 'gcc-11 -O2 -fcf-protection -Wa,--generate-missing-build-notes=yes'
+              default_cc: 'gcc-11 -fcf-protection -Wa,--generate-missing-build-notes=yes'
+              optflags: '-O2'
               LDFLAGS: '-Wl,-z,now'
               # FIXME: Drop skipping options
               # https://bugs.ruby-lang.org/issues/18061
               # https://sourceware.org/annobin/annobin.html/Test-pie.html
-              TEST_ANNOCHECK_OPTS: "--skip-pie"
+              TEST_ANNOCHECK_OPTS: "--skip-pie --skip-gaps"
             check: true
           - { name: clang-16,  env: { default_cc: clang-16 } }
           - { name: clang-15,  env: { default_cc: clang-15 } }