libc: Add memchr for cortex-a53

Change-Id: Id84695006991bb9aaef8ddc127c3bee646948527
Implement <pty.h>.

Based on the package/apps/Terminal implementation. I'll switch them over
shortly. This also lets us build the toybox version of netcat.

Change-Id: Ia922a100141a67409264b43b937eeca07b21f344