Review Request JDK-8205627: Replace hardcoded spec version value in build.tools.ExtLink taglet

[mandy chung]

http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8205627/webrev.00

ExtLink taglet generates an external link to a JDK documentation page of 
a specific release.  This patch replaces the hardcoded value with the 
value of a system property "extlink.spec.version" passed to the javadoc 
at runtime.  The taglet can't rely on Runtime.version().feature() 
because javadoc is running on the boot JDK.

Mandy