lib: sbi: Fix compile errors using -Os option

When compiling with -Os option along with -ffreestanding, both GCC
and clang will add implicit calls to memcpy() and memset() for stack
variables initialized in declaration.

The C standard as per Clause 4, the compiler cannot necessarily
assume that anything beyond:

 * float.h
 * iso646.h
 * limits.h
 * stdalign.h
 * stdarg.h
 * stdbool.h
 * stddef.h
 * stdint.h
 * stdnoreturn.h
 * fenv.h
 * math.h
 * and the numeric conversion functions of stdlib.h.

This patch maps memcpy() and memset() as weak-alias of sbi_memcpy()
and sbi_memset() respectively so that implicit calls to memcpy()
and memset() will compile properly.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Xiang W <wxjstz@126.com>