[PATCH][AArch64] Single thread lowlevellock optimization

[Szabolcs Nagy]

Do single thread lock optimization in aarch64 libc. Atomic operations
hurt the performance of some single-threaded programs using stdio
(usually getc/putc in a loop).

Ideally such optimization should be done at a higher level and in a
target independent way as in
https://sourceware.org/ml/libc-alpha/2017-05/msg00479.html
but that approach will need more discussion so do it in lowlevellocks,
similarly to x86, until there is consensus.

Differences compared to the current x86_64 behaviour:
- The optimization is not silently applied to shared locks, in that
case the build fails.
- Unlock assumes the futex value is 0 or 1, there are no waiters to
wake (that would not work in single thread and libc does not use
such locks, to be sure lll_cond* is undefed).

This speeds up a getchar loop about 2-4x depending on the cpu,
while only cause around 5-10% regression for the multi-threaded case
(other libc internal locks are not expected to be performance
critical or significantly affected by this change).

2017-06-16  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* sysdeps/unix/sysv/linux/aarch64/lowlevellock.h: New file.
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