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>
2dfbae2853