build-infra: Cannot use icedtea as boot for closed build.

The problem is in the generation of files for ct.sym. Here is the error message cut from a mail written by David Holms on build-infra-dev:

Here's the failing command (edited for presentation):

/usr/lib/jvm/java-7-openjdk-amd64/bin/java -Xms64M -Xmx1100M -XX:PermSize=32m -XX:MaxPermSize=160m -XX:ThreadStackSize=1536 \

-Xbootclasspath/a:/java/embedded/users/dh198349/build-infra/builds/b00/se-linux-i586-ea/jdk/classes \

"-Xbootclasspath/p:/java/embedded/users/dh198349/build-infra/builds/b00/se-linux-i586-ea/langtools/dist/bootstrap/lib/javac.jar" -jar /java/embedded/users/dh198349/build-infra/builds/b00/se-linux-i586-ea/langtools/dist/bootstrap/lib/javac.jar \
            -XDprocess.packages -proc:only \
            -processor com.sun.tools.javac.sym.CreateSymbols \

-Acom.sun.tools.javac.sym.Jar=/java/embedded/users/dh198349/build-infra/builds/b00/se-linux-i586-ea/images/lib/rt.jar \

-Acom.sun.tools.javac.sym.Dest=/java/embedded/users/dh198349/build-infra/builds/b00/se-linux-i586-ea/images/symbols/META-INF/sym/rt.jar \
            java.applet java.awt java.awt.color java.awt.datatransfer java.awt.dnd java.awt.event java.awt.font java.awt.geom java.awt.im java.awt.im.spi java.awt.image java.awt.image.renderable java.awt.print java.beans java.beans.beancontext java.io java.lang
...
com.sun.servicetag com.oracle.net com.oracle.nio
Error occurred during initialization of VM
java/lang/UnsupportedClassVersionError: sun/misc/PostVMInitHook : Unsupported major.minor version 52.0