RFR: 8039957: Replace the last few %p usages with PTR_FORMAT in the GC code

[Stefan Karlsson]

Please, review this small patch the GC code to replace the last few 
usages of %p with PTR_FORMAT.

http://cr.openjdk.java.net/~stefank/8039957/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8039957

One of the instances used %p to print a size_t. I decided to cast the 
value to pointer instead of using SIZE_FORMAT. I'll be happy to change 
it to SIZE_FORMAT or SIZE_FORMAT_HEX if anyone cares and has an opinion 
about this unused piece of code. :)

thanks,
StefanK