6873059: Explicitly use -source 6 -target 6 when compiling with the boot jdk javac

Summary: The bootstrap javac currently uses the default source and targets of the boot javac
Reviewed-by: ohair

make/common/shared/Defs-java.gmk

@@ -201,7 +201,10 @@ endif
 ifeq ($(JAVAC_WARNINGS_FATAL), true)
   BOOT_JAVACFLAGS  += -Werror
 endif
-BOOT_JAVACFLAGS  += -encoding ascii
+
+BOOT_SOURCE_LANGUAGE_VERSION = 6
+BOOT_TARGET_CLASS_VERSION = 6
+BOOT_JAVACFLAGS  += -encoding ascii -source $(BOOT_SOURCE_LANGUAGE_VERSION) -target $(BOOT_TARGET_CLASS_VERSION)
 BOOT_JAR_JFLAGS += $(JAR_JFLAGS)
 
 BOOT_JAVACFLAGS  += $(NO_PROPRIETARY_API_WARNINGS)