configure: Don't implicitly hardcode list of KVM architectures

configure: Don't implicitly hardcode list of KVM architectures

The code creating the symlink from linux-headers/asm to the
architecture specific linux-headers/asm-$arch directory was
implicitly hardcoding a list of KVM supporting architectures.
Add a default case for the common "Linux architecture name and
QEMU CPU name match" case, so future architectures will only
need to add code if they've managed to get mismatched names.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>