hg: jdk9/sandbox: Add AppendFile macro, and use it to create a dynamic spec file.

[magnus.ihse.bursie at oracle.com]

Changeset: e21b1932733c
Author:    ihse
Date:      2017-09-08 15:10 +0200
URL:       http://hg.openjdk.java.net/jdk9/sandbox/rev/e21b1932733c

Add AppendFile macro, and use it to create a dynamic spec file.

! make/Help.gmk
! make/RunTestsPrebuilt.gmk
! make/common/MakeBase.gmk