configure: arm64: Don't display 'aarch64' as the default architecture

--arch=aarch64, intentional or not, has been supported since the initial
arm64 support, commit 39ac3f84 ("arm64: initial drop"). However,
"aarch64" does not show up in the list of supported architectures, but
it's displayed as the default architecture if doing ./configure --help
on an arm64 machine.

Keep everything consistent and make sure that the default value for
$arch is "arm64", but still allow --arch=aarch64, in case they are users
that use this configuration for kvm-unit-tests.

The help text for --arch changes from:

   --arch=ARCH            architecture to compile for (aarch64). ARCH can be one of:
                           arm, arm64, i386, ppc64, riscv32, riscv64, s390x, x86_64

to:

    --arch=ARCH            architecture to compile for (arm64). ARCH can be one of:
                           arm, arm64, i386, ppc64, riscv32, riscv64, s390x, x86_64

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>