[OpenJDK 2D-Dev] RFR: 8074827: Resolve disabled warnings for libjavajpeg

[Philip Race]

https://bugs.openjdk.java.net/browse/JDK-8074827
http://cr.openjdk.java.net/~prr/8074827/

Passes JPRT.
gcc clobbered is left since that seems to be a gcc bug fixed in a later gcc

-phil.