KVM: selftests: Rename UNAME_M to ARCH_DIR, fill explicitly for x86

Rename UNAME_M to ARCH_DIR and explicitly set it directly for x86.  At
this point, the name of the arch directory really doesn't have anything
to do with `uname -m`, and UNAME_M is unnecessarily confusing given that
its purpose is purely to identify the arch specific directory.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>