arm: swscale: Only compile the rgb2yuv asm if .dn aliases are supported

Vanilla clang supports altmacro since clang 5.0, and thus doesn't
require gas-preprocessor for building the arm assembly any longer.

However, the built-in assembler doesn't support .dn directives.

This readds checks that were removed in d7320ca3, when
the last usage of .dn directives within libav were removed.

Alternatively, the assembly could be rewritten to not use the
.dn directive, making it available to clang users.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -2149,6 +2149,7 @@ SYSTEM_LIBRARIES="
 
 TOOLCHAIN_FEATURES="
     as_arch_directive
+    as_dn_directive
     as_fpu_directive
     as_func
     as_object_arch
@@ -5530,6 +5531,7 @@ EOF
     check_inline_asm asm_mod_q '"add r0, %Q0, %R0" :: "r"((long long)0)'
 
     check_as as_arch_directive ".arch armv7-a"
+    check_as as_dn_directive   "ra .dn d0.i16"
     check_as as_fpu_directive  ".fpu neon"
 
     # llvm's integrated assembler supports .object_arch from llvm 3.5

libswscale/arm/rgb2yuv_neon_16.S

@@ -18,6 +18,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "config.h"
+#if HAVE_AS_DN_DIRECTIVE
 #include "rgb2yuv_neon_common.S"
 
 /* downsampled R16G16B16 x8 */
@@ -78,3 +80,4 @@ alias_qw    c8x8x2, q10
 .endm
 
     loop_420sp  rgbx, nv12, init, kernel_420_16x2, 16
+#endif

libswscale/arm/rgb2yuv_neon_32.S

@@ -18,6 +18,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "config.h"
+#if HAVE_AS_DN_DIRECTIVE
 #include "rgb2yuv_neon_common.S"
 
 /* downsampled R16G16B16 x8 */
@@ -117,3 +119,4 @@ alias_qw    c8x8x2, q10
 
 
     loop_420sp  rgbx, nv12, init, kernel_420_16x2, 32
+#endif

libswscale/arm/swscale_unscaled.c

@@ -23,6 +23,7 @@
 #include "libswscale/swscale_internal.h"
 #include "libavutil/arm/cpu.h"
 
+#if HAVE_AS_DN_DIRECTIVE
 extern void rgbx_to_nv12_neon_32(const uint8_t *src, uint8_t *y, uint8_t *chroma,
                 int width, int height,
                 int y_stride, int c_stride, int src_stride,
@@ -178,3 +179,8 @@ void ff_get_unscaled_swscale_arm(SwsContext *c)
     if (have_neon(cpu_flags))
         get_unscaled_swscale_neon(c);
 }
+#else
+void ff_get_unscaled_swscale_arm(SwsContext *c)
+{
+}
+#endif