mips64: disable intercepting the spinlock function

This change disables intercepting pthread_spin_lock() for MIPS64, similar to
r13190 for MIPS32. Used in DRD and Helgrind as a workaround for the issue
#311690.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13417 a5019735-40e9-0310-863c-91ae7b9d1cf9