Add an OSX build with xcode12

.travis.yml

@@ -204,6 +204,17 @@ matrix:
       env:
         - BTYPE="TARGET=NEHALEM BINARY=64 INTERFACE64=1 FC=gfortran-8"
 
+    - <<: *test-macos
+      osx_image: xcode12
+      before_script:
+        - COMMON_FLAGS="DYNAMIC_ARCH=1 NUM_THREADS=32"
+        - brew update
+        - brew install gcc@10 # for gfortran
+      script:
+        - travis_wait 45 make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
+      env:
+        - BTYPE="TARGET=NEHALEM BINARY=64 INTERFACE64=1 FC=gfortran-10"
+        
     - <<: *test-macos
       osx_image: xcode10.0
       env: