net/relayd: fix build on armv6/armv7

Add armv6/armv7 to CLANG_ARCH so OpenBSD's bsd.own.mk knows that we are
building with clang on these.

Approved by: portmgr (build fix blanket)
MFH: 2023Q4

(cherry picked from commit b86fe5fa38e252506a13820b414c2453acf96bb8)