ia64: Remove custom lowlevellock.h

ia64 seems to use the same implementation of low-level locks as the
generic Linux lowlevellock.h.  The futex syscalls are somewhat
different, but Roland thought it shouldn't matter.  Note that the futex
calls are on the slow path always (except for PI mutexes).

Removing the custom low-level lock implementation will make further
refactoring easier, for example adding proper error checking to futex
operations.