Hello,

The attached switches the PPC architectures over to the new 
"return_value" gdbarch method.  Due to a lack of coverage in the 
testsuite, this change doesn't actually improve the existing test 
results (ppc64 GNU/Linux and ppc32 NetBSD).

Consequently, I wrote some new tests (will post in next few days) that 
beef up the testsuite and, with them, the results definitly improve!

Anyway, ok to commit?
Andrew