Automatically detect missing symbols

which are usually optimized away by -O3.

This CI can detect missing exports like
ea84a68075 which was needed for
761346a960.

.travis.yml

@@ -143,6 +143,8 @@ env:
     name: x86_64-linux
     os: linux
     <<: *gcc-8
+    env:
+      - TEST_MJIT_SYMBOLS=1 # detect exports missing for MJIT
 
   - &jemalloc
     name: --with-jemalloc

test/lib/jit_support.rb

@@ -29,6 +29,7 @@ module JITSupport
     ]
     args << '--jit-wait' if wait
     args << '--jit-save-temps' if save_temps
+    args << '--jit-debug' if ENV['TEST_MJIT_SYMBOLS'] == '1'
     args << '-e' << script
     base_env = { 'MJIT_SEARCH_BUILD_DIR' => 'true' } # workaround to skip requiring `make install` for `make test-all`
     if preloadenv = RbConfig::CONFIG['PRELOADENV'] and !preloadenv.empty?