An optimized memchr was missing for AArch64. This version is similar to

strchr and is significantly faster than the C version.

2016-11-04  Wilco Dijkstra  <wdijkstr@arm.com>
	    Kevin Petit  <kevin.petit@arm.com>

	* sysdeps/aarch64/memchr.S (__memchr): New file.