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>

libswscale/utils.c

@@ -1044,9 +1044,10 @@ int sws_setColorspaceDetails(struct SwsContext *c, const int inv_table[4],
                                  contrast, saturation);
         // FIXME factorize
 
-        if (ARCH_PPC)
-            ff_yuv2rgb_init_tables_ppc(c, inv_table, brightness,
-                                       contrast, saturation);
+#if ARCH_PPC
+        ff_yuv2rgb_init_tables_ppc(c, inv_table, brightness,
+                                   contrast, saturation);
+#endif
     }
 
     fill_rgb2yuv_table(c, table, dstRange);