configure: Explicitly set ARFLAGS so we can build with GNU Make 4.0

configure: Explicitly set ARFLAGS so we can build with GNU Make 4.0

Our rules.mak adds '-rR' to MAKEFLAGS to indicate that we will be
explicitly specifying everything and not relying on any default
variables or rules. However we were accidentally relying on the
default ARFLAGS ("rv"). This went unnoticed because of a bug in
GNU Make 3.82 and earlier which meant that adding -rR to MAKEFLAGS
only affected submakes, not the currently running instance.
Explicitly set ARFLAGS in config-host.mak, in the same way we
handle CFLAGS and LDFLAGS; this will allow us to work with
Make 4.0.

Thanks to Paul Smith for analyzing this bug for us.

Cc: qemu-stable@nongnu.org
Reported-by: Ken Moffat <zarniwhoop@ntlworld.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>