riscv: Enable seccomp architecture tracking

To enable seccomp constant action bitmaps, we need to have a static
mapping to the audit architecture and system call table size. Add these
for riscv.

Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/58ef925d00505cbb77478fa6bd2b48ab2d902460.1605101222.git.yifeifz2@illinois.edu