RFR: 8011372: Remove -p from cp in IdleCompilation.gmk

[Erik Joelsson]

In IdlCompilation.gmk there is a cp line with the -p flag. I can see no 
apparent reason for this flag and it was reported that it causes 
problems when building over NFS in certain situations. This patch 
removes the flag.

http://cr.openjdk.java.net/~erikj/8011372/webrev.01/
/Erik