configure: Replace bash code by standard shell code

"+=" does not work with dash and other simple /bin/sh implementations.

The new code prepends the flag while the old code either did not work
(it continued after an error message which typically was not read) or
appended the flag. That difference should not matter here.
Reported-by: NOlaf Hering <olaf@aepfle.de>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -2812,7 +2812,7 @@ int main(int argc, char **argv)
 }
 EOF
   if ! compile_prog "" "" ; then
-    CFLAGS+="-march=i486"
+    CFLAGS="-march=i486 $CFLAGS"
   fi
 fi