<AWT Dev> RFR: 8155784: Build failure on Linux arm64

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8155784
Fix: http://cr.openjdk.java.net/~prr/8155784/

There are two distinct build failures on cross-compilation on Linux 
Embedded platforms for ARM.
This resolves both of these.

One is due to no longer directly including keysym.def

The other is due to using "NULL" as "0" causing a warning treated as error.
This was already fixed separately but since this was fixed on Friday 
after 9-client
was already pushed to 9-dev we need to separately fix it there since 
only 9-dev
builds these embedded platforms.

jprt was used to verify the fix.

-phil.