RFR: 8003310: Enable -Wunused when compiling with GCC

[Mikael Vidstedt]

All,

Please review the below change. The change adds the -Wunused flag when 
compiling with GCC and removes a number of unused functions/dead code.

In the process I found one function (same_page) which was duplicated in 
four different places. I merged it to a single function, renamed it to 
clamp_address_in_page, added some comments and refactored it to be 
slightly easier to understand. I also added unit tests for it. Feedback 
appreciated (especially on the name).

http://cr.openjdk.java.net/~mikael/8003310/webrev.00/

Passes JPRT and the built-in unit tests (-XX:+ExecuteInternalVMTests).

Thanks,
Mikael