Remove x86_64 specific lowlevellock/cancellation

The x86_64 specific implemention has CFI directives like
`.cfi_adjust_cfa_offset 128` which are incorrect when RBP is used as the
canonical frame address.

This follows the spirit of the following two commits by removing the
x86_64 specific implementation. The generic implementation will be used.

* eb76e5b4 ("nptl: Reinstate pthread_timedjoin_np as a cancellation point (BZ#24215)")
* c50e1c26 ("x86: Remove arch-specific low level lock implementation")