hg: build-infra/jdk9/hotspot: Put precompiled headers in a separate directory.

[magnus.ihse.bursie at oracle.com]

Changeset: b3106173d502
Author:    ihse
Date:      2015-10-08 14:00 +0200
URL:       http://hg.openjdk.java.net/build-infra/jdk9/hotspot/rev/b3106173d502

Put precompiled headers in a separate directory.

! makefiles/Common.gmk
! makefiles/CompileJvm.gmk