alpha: select CONFIG_ARCH_USE_CMPXCHG_LOCKREF.

On Alpha we have spinlocks that are 32b in size and an efficient
cmpxchg64 implementation, so we qualify to make use of cmpxchg backed
lockrefs. Select the ARCH_USE_CMPXCHG_LOCKREF Kconfig symbol and provide
a trivial implementation of arch_spin_value_unlocked to satisfy the
lockref code.

Using Linus' simple testcase from
http://article.gmane.org/gmane.linux.file-systems/77466 on a dual CPU
ES47 system I see around an 8% gain:

    N       Min       Max    Median       Avg      Stddev
x  30   6194580   6295654   6272504   6272514   17694.232
+  30   6731164   6786334   6767982d   6764274   13738.863
Difference at 95.0% confidence
        491760 +/- 8188.17
        7.83992% +/- 0.130541%
        (Student's t, pooled s = 15840.5)
Signed-off-by: NMatt Turner <mattst88@gmail.com>

arch/alpha/Kconfig

@@ -3,6 +3,7 @@ config ALPHA
 	default y
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
+	select ARCH_USE_CMPXCHG_LOCKREF
 	select HAVE_AOUT
 	select HAVE_IDE
 	select HAVE_OPROFILE

arch/alpha/include/asm/spinlock.h

@@ -16,6 +16,11 @@
 #define arch_spin_unlock_wait(x) \
 		do { cpu_relax(); } while ((x)->lock)
 
+static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
+{
+        return lock.lock == 0;
+}
+
 static inline void arch_spin_unlock(arch_spinlock_t * lock)
 {
 	mb();