[minor] [build] Set java options when generating mima ignores.

The default java options make the call to GenerateMIMAIgnore take
forever to run since it's gc'ing all the time. Improve things by
setting the perm gen size / max heap size to larger values.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes apache#5615 from vanzin/gen-mima-fix and squashes the following commits:

f44e921 [Marcelo Vanzin] [minor] [build] Set java options when generating mima ignores.

dev/mima

@@ -27,16 +27,21 @@ cd "$FWDIR"
 echo -e "q\n" | build/sbt oldDeps/update
 rm -f .generated-mima*
 
+generate_mima_ignore() {
+  SPARK_JAVA_OPTS="-XX:MaxPermSize=1g -Xmx2g" \
+    ./bin/spark-class org.apache.spark.tools.GenerateMIMAIgnore
+}
+
 # Generate Mima Ignore is called twice, first with latest built jars
 # on the classpath and then again with previous version jars on the classpath.
 # Because of a bug in GenerateMIMAIgnore that when old jars are ahead on classpath
 # it did not process the new classes (which are in assembly jar).
-./bin/spark-class org.apache.spark.tools.GenerateMIMAIgnore
+generate_mima_ignore
 
 export SPARK_CLASSPATH="`find lib_managed \( -name '*spark*jar' -a -type f \) | tr "\\n" ":"`"
 echo "SPARK_CLASSPATH=$SPARK_CLASSPATH"
 
-./bin/spark-class org.apache.spark.tools.GenerateMIMAIgnore
+generate_mima_ignore
 
 echo -e "q\n" | build/sbt mima-report-binary-issues | grep -v -e "info.*Resolving"
 ret_val=$?