1. add lib_fork api in libs/libc, we need a fork() api to implement the
fork relative method, such as pthread_atfork
2. rename the assembly fork entry function name to up_fork(), and rename
the up_fork() to arch specific name, such as
sim_fork()/arm_fork()/mips_fork() etc.
Signed-off-by: guoshichao <guoshichao@xiaomi.com>