x86: relocate get/set debugreg fcns to include/asm/debugreg.

Since we already have a debugreg.h header file, move the
assoc. get/set functions to it.  In addition to it being the
logical home for them, it has a secondary advantage.  The
functions that are moved use BUG().  So we really need to
have linux/bug.h in scope.  But asm/processor.h is used about
600 times, vs. only about 15 for debugreg.h -- so adding bug.h
to the latter reduces the amount of time we'll be processing
it during a compile.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: "H. Peter Anvin" <hpa@zytor.com>