[Feature] riscv32 rt-thread支持跑在s-mode

[zhuzhizhan]

Describe problem solved by the proposed feature

riscv32 rt-thread支持跑在s-mode

Describe your preferred solution

riscv32 rt-thread支持跑在s-mode

Describe possible alternatives

riscv32 rt-thread支持跑在s-mode

[zhuzhizhan]

目前riscv32 rt-thread只跑在m-mode，可否考虑支持riscv32 rt-thread跑在s-mode.