Add ifdef HAVE_NEON around the contents of arm_arch_common_macro.S

This file is built on its own from within the xcode projects,
even though it isn't necessary. Previously its contents was just
empty, but now a .syntax unified was added, which failed the build
when building for arm64.

Make this file a no-op, just like the other arm assembly source files,
unless HAVE_NEON is defined.

Review at https://rbcommons.com/s/OpenH264/r/757/.