RFR: JDK-8199451 Create linux-aarch64 cross-compilation devkit, and fix cross-compilation

[Magnus Ihse Bursie]

While Oracle no longer actively supports linux-aarch64, it is good to 
keep the build functioning for cross-compilation.

I've created a devkit for cross-compilation from linux-x64 to 
linux-aarch64, and fixed a recently introduced bug with buildjdk 
SYSROOT_CFLAGS.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199451
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8199451-create-linux-aarch64-devkit/webrev.01

/Magnus