[SPARK-20546][DEPLOY] spark-class gets syntax error in posix mode

## What changes were proposed in this pull request?

Updated spark-class to turn off posix mode so the process substitution doesn't cause a syntax error.

## How was this patch tested?

Existing unit tests, manual spark-shell testing with posix mode on

Author: jyu00 <jessieyu@us.ibm.com>

Closes #17852 from jyu00/master.

bin/spark-class

@@ -72,6 +72,8 @@ build_command() {
   printf "%d\0" $?
 }
 
+# Turn off posix mode since it does not allow process substitution
+set +o posix
 CMD=()
 while IFS= read -d '' -r ARG; do
   CMD+=("$ARG")