target/riscv: add common magic

target/riscv: add common magic

Add common_magic member to struct riscv_info.
Introduce is_riscv() helper.

Change-Id: I1af05988ad869342ba5dc6d4d0ba0ec6a8bf7bc7
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6999
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tim Newsome <tim@sifive.com>