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 d7320ca3ed, 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