Avoid string conversion in PosixException constructor

Author: otethal

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/PosixSupportLibrary.java

@@ -875,7 +875,6 @@ public static class PosixException extends Exception {
         private final TruffleString msg;
 
         public PosixException(int errorCode, TruffleString message) {
-            super(message.toJavaStringUncached());
             this.errorCode = errorCode;
             msg = message;
         }
@@ -884,6 +883,11 @@ public final TruffleString getMessageAsTruffleString() {
             return msg;
         }
 
+        @Override
+        public String getMessage() {
+            return msg.toJavaStringUncached();
+        }
+
         public int getErrorCode() {
             return errorCode;
         }