ARM uses compiler intrinsics, so don't require GNU as.

Index: build/cmake/aom_configure.cmake
--- build/cmake/aom_configure.cmake.orig
+++ build/cmake/aom_configure.cmake
@@ -194,6 +194,10 @@ elseif(AOM_TARGET_CPU MATCHES "arm")
     if(NOT CMAKE_ASM_COMPILER)
       set(CMAKE_ASM_COMPILER ${CMAKE_C_COMPILER} "-c -mimplicit-it=always")
     endif()
+  elseif(AOM_TARGET_SYSTEM STREQUAL "OpenBSD")
+    if(NOT CMAKE_ASM_COMPILER)
+      set(CMAKE_ASM_COMPILER ${CMAKE_C_COMPILER})
+    endif()
   else()
     if(NOT CMAKE_ASM_COMPILER)
       set(CMAKE_ASM_COMPILER as)
