lib: string: add memcpy and memset

Introduce string utilities from tag v0.9.14
of MUSL libc project(http://www.musl-libc.org/) with the following
modifications,

1. Remove unused ports and functions
2. Tweak for F9 Microkernel environment

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Merge pull request #57 from vh21/lib-str

lib: string: add memcpy and memset