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
@@ -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
