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.
Merge pull request #1280 from mstorsjo/arm-common-ifdefs

Add ifdef HAVE_NEON around the contents of arm_arch_common_macro.S