Use mutex hint bit in PPC LWARX instructions, where possible.

The hint bit makes for a small but measurable performance improvement
in access to contended spinlocks.

On the other hand, some PPC chips give an illegal-instruction failure.
There doesn't seem to be a completely bulletproof way to tell whether the
hint bit will cause an illegal-instruction failure other than by trying
it; but most if not all 64-bit PPC machines should accept it, so follow
the Linux kernel's lead and assume it's okay to use it in 64-bit builds.
Of course we must also check whether the assembler accepts the command,
since even with a recent CPU the toolchain could be old.

Patch by Manabu Ori, significantly modified by me.

src/include/storage/s_lock.h

@@ -372,7 +372,11 @@ tas(volatile slock_t *lock)
 	int _res;
 
 	__asm__ __volatile__(
+#ifdef USE_PPC_LWARX_MUTEX_HINT
+"	lwarx   %0,0,%3,1	\n"
+#else
 "	lwarx   %0,0,%3		\n"
+#endif
 "	cmpwi   %0,0		\n"
 "	bne     1f			\n"
 "	addi    %0,%0,1		\n"