mips64: use generic MK_(guess_and_register_stack)

Follow up to Philippe's change in r14392 which does a cleanup and makes
all architectures use the same code to guess and register stack.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14490 a5019735-40e9-0310-863c-91ae7b9d1cf9