[11] Review Request JDK-8206184: docs-reference build fails due to extlink.spec.version property not set

[mandy chung]

"docs-reference" target fails with `extlink.spec.version` system
property not set.  It's a regression caused by JDK-8205627.
docs-reference is built with BootJDK javadoc that also uses
ExtLink taglet and so make/Docs.gmk needs to be adjusted to set
the system property in $JAVADOC_CMD.

The fix is straight-forward (thanks to Erik for suggesting
this better fix).

http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8206184/webrev.00/

Mandy