Avoid duplicate object files during the link. There are legitimate
cases where a link command-line would include duplicate object files
because two independent subsystems both depend on common infrastructure.
Use GNU make's $(sort) function to remove duplicate object files from
the link command-line.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>