libcpu: riscv: rv64: fixed warnings

When building bsp/cvitek/c906_little, compiler reports:

```
.../rt-thread/libcpu/risc-v/rv64/trap.c:
In function 'handle_trap':
.../rt-thread/libcpu/risc-v/rv64/trap.c:106:13:
warning: implicit declaration of function 'rt_hw_tick_isr';
did you mean 'rt_hw_stack_init'? [-Wimplicit-function-declaration]
  106 |             rt_hw_tick_isr();
      |             ^~~~~~~~~~~~~~
      |             rt_hw_stack_init
.../rt-thread/libcpu/risc-v/rv64/trap.c:110:13:
warning: implicit declaration of function 'rt_hw_irq_isr';
did you mean 'rt_hw_soft_irq_isr'? [-Wimplicit-function-declaration]
  110 |             rt_hw_irq_isr();
      |             ^~~~~~~~~~~~~
      |             rt_hw_soft_irq_isr
```

rt_hw_tick_isr()/rt_hw_irq_isr() are implemented by bsp, but
libcpu/risc-v/rv64 doesn't declare them, so compiler warns.

There are three BSPs using 'rv64' (libcpu/risc-v/rv64):
- `bsp/cvitek/c906_little/rtconfig.py`
- `bsp/juicevm/rtconfig.py`
- `bsp/k210/rtconfig.py`

`handle_trap` in `libcpu/risc-v/rv64` is defined as weak.
BSP can use this function directly or define and overload
it by itself.
If bsp use this function directly, bsp need to pay
attention to the fact that three functions will be called
in this function:

- `rt_hw_soft_irq_isr`
- `rt_hw_tick_isr`
- `rt_hw_irq_isr`

In `libcpu/risc-v/rv64`, `rt_hw_soft_irq_isr` has a weak
definition, while the other two do not. This means that
if the bsp does not overload `handle_trap`, bsp must
define `rt_hw_tick_isr` and `rt_hw_irq_isr` itself.
This is also the practice of `bsp/cvitek/c906_little`.
There is also a similar bsp `bsp/k210`, and the form of
`bsp/juicevm` implements `handle_trap` by itself.

It seems that `rt_hw_tick_isr` and `rt_hw_irq_isr` are
not required to be implemented by all BSPs using
`libcpu/risc-v/rv64`. The premise for BSP to implement
them is that it does not overload `handle_trap`. So
declaring `rt_hw_tick_isr` and `rt_hw_irq_isr` with
extern in `libcpu/risc-v/rv64` is not proper.

In addition, the `rt_hw_tick_isr/rt_hw_irq_isr` are only
used by `libcpu/risc-v/rv64`, so it is not worth putting
the declaration in `./include/rthw.h`.

Sum up, the best solution is to add weak definition to
`rt_hw_tick_isr/rt_hw_irq_isr` as existing `rt_hw_soft_irq_isr`.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

libcpu/risc-v/rv64/trap.c

@@ -92,6 +92,15 @@ rt_weak void rt_hw_soft_irq_isr(void)
 
 }
 
+rt_weak int rt_hw_tick_isr(void)
+{
+    return 0;
+}
+
+rt_weak void rt_hw_irq_isr(void)
+{
+
+}
 
 rt_weak rt_size_t handle_trap(rt_size_t cause, rt_size_t epc, rt_size_t *sp)
 {