An optimized memchr was missing for AArch64. This version is similar to strchr and is significantly
faster than the C version. Passes GLIBC tests.

OK for commit?

ChangeLog:
2015-09-25  Wilco Dijkstra  <wdijkstr@arm.com>
2015-09-25  Kevin Petit  <kevin.petit@arm.com>

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