Doc: add "module swap PrgEnv-nvidia PrgEnv-gnu" when building on Polaris

Default loaded module is PrgEnv-nvidia on Polaris. If not swapped to
PrgEnv-gnu, all Fortran test programs failed when running
darhan-test/regression/run-all.sh. Only C and C++ programs ran fine.

According to Phil, Darshan is usually built with gnu compilers on DOE
parallel computers, because then the library can be safely linked into
executables built with any compiler.

Author: wkliao

darshan-test/RELEASE-CHECKLIST.txt

@@ -31,13 +31,14 @@ Notes on how to release a new version of Darshan
     the darshan-logs repo in the darshan_logs/release_logs/ directory
     a. log into polaris.alcf.anl.gov
     b. clone darshan repo, say into $HOME/Darshan/Github/darshan
-    c. install Darshan, say into folder $HOME/Darshan/Github/DEV
-    d. cd $HOME/Darshan/Github/darshan/darshan-test/regression/
-    e. ./run-all.sh $HOME/Darshan/Github/DEV $HOME/Darshan/TEST alcf-polaris-cray-module
-    f. clone darshan-logs repo, say into $HOME/Darshan/Github/darshan-logs
-    g. cd $HOME/Darshan/Github/darshan-logs/darshan_logs/release_logs
-    h. cp $HOME/Darshan/TEST/mpi-io-test.darshan mpi-io-test-x86_64-3.5.0.darshan
-    i. Create a new PR
+    c. module swap PrgEnv-nvidia PrgEnv-gnu
+    d. install Darshan, say into folder $HOME/Darshan/Github/DEV
+    e. cd $HOME/Darshan/Github/darshan/darshan-test/regression/
+    f. ./run-all.sh $HOME/Darshan/Github/DEV $HOME/Darshan/TEST alcf-polaris-cray-module
+    g. clone darshan-logs repo, say into $HOME/Darshan/Github/darshan-logs
+    h. cd $HOME/Darshan/Github/darshan-logs/darshan_logs/release_logs
+    i. cp $HOME/Darshan/TEST/mpi-io-test.darshan mpi-io-test-x86_64-3.5.0.darshan
+    j. Create a new PR
 10) make a tag for the release according to instructions at
     http://git-scm.com/book/en/Git-Basics-Tagging
     - example (annotated tag, pushed to repo):