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 30ee5d17, gcc builds continued
implementing 64-bit atomics by way of intrinsics.  Back-patch to v13,
where that commit first appeared.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20201011051043.GA1724101@rfd.leadboat.com

src/include/port/atomics/arch-ppc.h

@@ -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