fix(libc): remove __putchar alias

fix(libc): remove __putchar alias

This issue was triggered by sparse tool:
lib/libc/putchar.c:9:5: warning:
 symbol '__putchar' was not declared. Should it be static?
Instead of setting __putchar as static, just remove the function and
directly use putchar() with a weak attribute.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ib35e4ba064f06010851bb860269b08462fe3d3bd