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>
