KVM: selftests: Override ARCH for x86_64 instead of using ARCH_DIR

Now that KVM selftests uses the kernel's canonical arch paths, directly
override ARCH to 'x86' when targeting x86_64 instead of defining ARCH_DIR
to redirect to appropriate paths.  ARCH_DIR was originally added to deal
with KVM selftests using the target triple ARCH for directories, e.g.
s390x and aarch64; keeping it around just to deal with the one-off alias
from x86_64=>x86 is unnecessary and confusing.

Note, even when selftests are built from the top-level Makefile, ARCH is
scoped to KVM's makefiles, i.e. overriding ARCH won't trip up some other
selftests that (somehow) expects x86_64 and can't work with x86.

Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-17-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>