mips64: Add an extra case for mips64 in ML_(get_CFA).

This patch resolves the issue with the memcheck/tests/dw4 on mips64 platform.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13895 a5019735-40e9-0310-863c-91ae7b9d1cf9