Add gitlab ci

.gitlab-ci.yml

+variables:
+    GIT_SUBMODULE_STRATEGY: recursive
+
+stages:
+  - sim
+
+sim:
+  stage: sim
+  tags:
+    - linux
+    - efinity
+  script:
+    - source init_env.sh
+    - make sim
+    - cd sim/cocotb
+    - make -j`nproc`
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