powerpc/barrier: Use CONFIG_PPC64 for barrier selection

Currently we use ifdef __powerpc64__ in barrier.h to decide if we
should use lwsync or eieio for SMPWMB which is then used by
__smp_wmb().

That means when we are building the compat VDSO we will use eieio,
because it's 32-bit code, even though we're building a 64-bit kernel
for a 64-bit CPU.

Although eieio should work, it would be cleaner if we always used the
same barrier, even for the 32-bit VDSO.

So change the ifdef to CONFIG_PPC64, so that the selection is made
based on the bitness of the kernel we're building for, not the current
compilation unit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201126131006.2431205-5-mpe@ellerman.id.au

arch/powerpc/include/asm/barrier.h

@@ -40,7 +40,7 @@
 #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
 
 /* The sub-arch has lwsync */
-#if defined(__powerpc64__) || defined(CONFIG_PPC_E500MC)
+#if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
 #    define SMPWMB      LWSYNC
 #else
 #    define SMPWMB      eieio