RFR(XS): 8151987: jexec should be executable

[Volker Simonis]

Hi,

can I please have a review for the following small fix:

http://cr.openjdk.java.net/~simonis/webrevs/2016/8151987/
https://bugs.openjdk.java.net/browse/JDK-8151987

'jexec' is currently not executable in the created images (i.e.
images/{jdk,jre}/lib/jexec.

There is currently a special handling in ImageBuilder for
'jspawnhelper' which apparently had the same problem.

This change extends this handling to 'jexec' as well.

Regards,
Volker