lib: add string functions from musl libc

1. Add string utilities from http://www.musl-libc.org/ with tag v0.9.14
2. Remove unused ports and functions
3. Tweak for F9 Microkernel environment
4. Currently only memcpy, memset, and strcmp is included

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>