For ppc gcc, implement 64-bit compare_exchange and fetch_add with asm.
While xlc defines __64BIT__, gcc does not.  Due to this oversight in
commit gcc builds continued
implementing 64-bit atomics by way of intrinsics.  Back-patch to v13,
where that commit first appeared.
Discussion:
@@ -32,14 +32,14 @@ typedef struct pg_atomic_uint32
} pg_atomic_uint32;
/* 64bit atomics are only supported in 64bit mode */
-#ifdef __64BIT__
+#if SIZEOF_VOID_P >= 8
#define PG_HAVE_ATOMIC_U64_SUPPORT
typedef struct pg_atomic_uint64
{
volatile uint64 value pg_attribute_aligned(8);
} pg_atomic_uint64;
-#endif /* __64BIT__ */
+#endif
/*
* This mimics gcc __atomic_compare_exchange_n(..., __ATOMIC_SEQ_CST), but
