Lock elision in the GNU C library

The main advantage of HLE is compatibility, that you don't need a separate code path for elision. But it requires no extra writes to the lock cache line for successful elision. glibc always writes to the lock's owner field in the default code path, so you would need a separate code path anyways to disable that.

With a separate code path (lock type) it's possible to use HLE. I did it, but removed that support later because it was redundant with RTM and RTM is better here because it's more flexible.