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 <[email protected]>

0 files changed, 0 insertions, 0 deletions