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>

libavcodec/celp_math.c

@@ -122,6 +122,7 @@ void ff_celp_math_init(CELPMContext *c)
 {
     c->dot_productf   = ff_dot_productf;
 
-    if(HAVE_MIPSFPU)
-        ff_celp_math_init_mips(c);
+#if HAVE_MIPSFPU
+    ff_celp_math_init_mips(c);
+#endif
 }