all: Replace if (ARCH_FOO) checks by #if ARCH_FOO

This is more spec-compliant because it does not rely
on dead-code elimination by the compiler. Especially
MSVC has problems with this, as can be seen in
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/296373.html
or
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/297022.html

This commit does not eliminate every instance where we rely
on dead code elimination: It only tackles branching to
the initialization of arch-specific dsp code, not e.g. all
uses of CONFIG_ and HAVE_ checks. But maybe it is already
enough to compile FFmpeg with MSVC with whole-programm-optimizations
enabled (if one does not disable too many components).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libswresample/rematrix.c

@@ -565,8 +565,9 @@ av_cold int swri_rematrix_init(SwrContext *s){
         s->matrix_ch[i][0]= ch_in;
     }
 
-    if(HAVE_X86ASM && HAVE_MMX)
-        return swri_rematrix_init_x86(s);
+#if ARCH_X86 && HAVE_X86ASM && HAVE_MMX
+    return swri_rematrix_init_x86(s);
+#endif
 
     return 0;
 }