libcpu: riscv: common: fixed build warnings

When building bsp/cvitek/c906_little, compiler warns:

```
.../rt-thread/libcpu/risc-v/common/trap_common.c:
In function 'rt_hw_interrupt_install':
.../rt-thread/libcpu/risc-v/common/trap_common.c:50:11:
warning: unused variable 'user_param' [-Wunused-variable]
   50 |     void *user_param = param;
      |           ^~~~~~~~~~
.../rt-thread/libcpu/risc-v/common/trap_common.c:
In function 'rt_rv32_system_irq_handler':
.../rt-thread/libcpu/risc-v/common/trap_common.c:77:25:
warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
   77 |         s_stack_frame = (rt_hw_stack_frame_t *)mscratch;
      |                         ^
```

Fixed these warnings as per indication from gcc.

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

libcpu/risc-v/common/trap_common.c

@@ -47,7 +47,6 @@ rt_weak rt_isr_handler_t rt_hw_interrupt_install(int vector, rt_isr_handler_t ha
         void *param, const char *name)
 {
     rt_isr_handler_t old_handler = RT_NULL;
-    void *user_param = param;
 
     if(vector < ISR_NUMBER)
     {
@@ -74,7 +73,7 @@ rt_weak void rt_rv32_system_irq_handler(rt_uint32_t mcause)
     rt_uint32_t exception = !(mcause & 0x80000000);
     if(exception)
     {
-        s_stack_frame = (rt_hw_stack_frame_t *)mscratch;
+        s_stack_frame = (volatile rt_hw_stack_frame_t *)(uintptr_t)mscratch;
         rt_show_stack_frame();
     }
     else