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.
This is the default when building with the clang built-in assembler,
but not if using the external assembler - thus always specify it,
for clarity.
Also use the three-operand for of a sub instruction in BS_NZC_CHECK.
The same is already done in the gnu version of the macro.
This fixes building most of the arm assembly with Apple's external
assembler. While this isn't a necessary goal in itself, there's no
harm in doing this either.
