arch: riscv: reset global pointer on exception

Reset the gp on exception entry from u-mode to protect the kernel
against a possible rogue user thread.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
Signed-off-by: Yong Cong Sin <ycsin@meta.com>
(cherry picked from commit e30db2d5)