arch: riscv: implement ARCH_HAS_CUSTOM_CURRENT_IMPL with GP

Implement `arch_curr_thread()` & `arch_set_curr_thread()`
with the global pointer (GP) register.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>