syscfg: Add a riscv64 architecture.

* src/syscfg/lock-obj-pub.riscv64-unknown-linux-gnu.h: New.
* src/Makefile.am (lock_obj_pub): Add it.

• Debian-bug-id: #891663
• Co-authored-by: Karsten Merker <merker@debian.org>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>