tests: arch: riscv: make sure that `gp` reg can't be corrupted

Add a test to make sure that the `gp` global pointer register used for
relative addressing when `CONFIG_RISCV_GP` is enabled can't be
corrupted by a rogue user thread.

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