csky: Use in_syscall & forget_syscall instead of r11_sig

We could use regs->sr 16-24 bits to detect syscall: VEC_TRAP0 and
r11_sig is no necessary for current implementation.

In this patch, we implement the in_syscall and forget_syscall which are
inspired from arm & nds32, but csky pt_regs has no syscall_num element
and we just set zero to regs->sr's vector-bits-field instead.

For ret_from_fork, current task was forked from parent which is in syscall
progress and its regs->sr has been already setted with VEC_TRAP0. See:
arch/csky/kernel/process.c: copy_thread()

Signed-off-by: Guo Ren <ren_guo@c-sky.com>