RFR: JDK-8182733 aarch64 build documentation misleading

[Magnus Ihse Bursie]

The build documentation misleadingly states that it is not recommended 
to build without cross compilation on aarch64. This patch updates the 
documentation. (I apologize for taking so long to address this. :-()

Bug: https://bugs.openjdk.java.net/browse/JDK-8182733
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8182733-update-aarch64-build-docs/webrev.01

/Magnus