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

crypto/chacha/asm/chacha-armv4.pl

@@ -172,8 +172,10 @@ $code.=<<___;
 #include "arm_arch.h"
 
 .text
-#if defined(__thumb2__)
+#if defined(__thumb2__) || defined(__clang__)
 .syntax	unified
+#endif
+#if defined(__thumb2__)
 .thumb
 #else
 .code	32