Fix chacha-armv4.pl with clang -fno-integrated-as.

The __clang__-guarded #defines cause gas to complain if clang is passed
-fno-integrated-as. Emitting .syntax unified when those are used fixes
this. This matches the change made to ghash-armv4.pl in
6cf412c4

.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/3694)