Refactor update_kernel_sp function

[LuernOutOfOrder]

This pull request refactors how the kernel stack pointer is updated in the RISC-V32 architecture. The main change is to make the stack pointer update function accept an explicit stack pointer argument, improving clarity and modularity. The update also reorganizes related code into more appropriate modules.

Refactoring and modularization of kernel stack pointer update:

• Added a new update_kernel_sp(sp: usize) function to src/arch/riscv32/mem.rs, which sets the kernel stack pointer using an explicit argument.
• Replaced the old update_kernel_sp() function in src/mem/mod.rs with a new mem_update_kernel_sp() function, which retrieves the stack pointer from memory and calls the new update_kernel_sp(sp) function.
• Updated boot process in src/boot.rs to use mem_update_kernel_sp() instead of the previous update_kernel_sp(). [1] [2]
• Moved the new memory-related code into the arch::riscv32::mem module and updated imports accordingly. [1] [2]

This pull-request improve the codebase by avoiding that the kernel function use arch specific asm, leave the asm in the arch specific module.