cmake: fix building luaL_loadbuffer_proto on non-aarch64

Commit 9294a95 ("cmake: disable luaL_loadbuffer_proto on OSS
Fuzz/aarch64") accidentally disabled test `luaL_loadbuffer_proto` on
non-aarch64 systems. The patch simplifies CMake condition and fixes
building of the test on non-aarch64 systems.

Follows up #72

tests/capi/CMakeLists.txt

@@ -124,7 +124,9 @@ endforeach()
 
 # OSS Fuzz build is broken on aarch64,
 # https://github.com/ligurio/lua-c-api-tests/issues/72
-if (NOT OSS_FUZZ AND NOT HW_ARCH STREQUAL "aarch64")
+if (OSS_FUZZ AND HW_ARCH STREQUAL "aarch64")
+  message(STATUS "luaL_loadbuffer_proto_test is disabled (issue #72)")
+else ()
   include(ProtobufMutator)
   add_subdirectory(luaL_loadbuffer_proto)
 endif ()