RFR: 8062808: Turn on the -Wreturn-type warning

[Stefan Karlsson]

Hi all,

I propose that we turn on the -Wreturn-type warning when compiling 
HotSpot with GCC.

This will help us catch missing return statements earlier in the 
development cycle.

http://cr.openjdk.java.net/~stefank/8062808/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8062808

thanks,
StefanK