When arm 64-bit atomic ops are available, define ARM_HAVE_ATOMIC64.  Use
that symbol (which will be correct in both kernel and userland contexts)
rather than just __arm__ to decide whether to use a local implementation.
Add vt(4) support to sparc64. The only driver currently present (ofwfb)
provides support for a variety of low-end graphics hardware (SBus adapters,
Mach64, QEMU's framebuffer, XVR-100). A driver for at least the Creator3D
cards will have to be present before this can become the default console
driver.

To test vt(4) on sparc64, set kern.vty=vt at the loader prompt.