kbuild: "PREEMPT" in UTS_VERSION

From: Matt Mackall <mpm@selenic.com>

Add PREEMPT to UTS_VERSION where enabled as is done for SMP to make
preempt kernels easily identifiable.
Added SMP PREEMPT as comment in compile.h to force it to be
updated when they change (sam).

Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>