configure: -march=i486 belongs in QEMU_CFLAGS, not CFLAGS

The distinction between QEMU_CFLAGS and CFLAGS is that the
former is for flags without which QEMU can't compile, whereas
the latter is for flags like "-g -O2" which the user can
safely override. "-march=i486" is in the former category, and
so belongs in QEMU_CFLAGS.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

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