For PowerPC instruction "addi", use constraint "b".

Without "b", a variant of the tas() code miscompiles on macOS 10.4.
This may also fix a compilation failure involving macOS 10.1.  Today's
compilers have been allocating acceptable registers with or without this
change, but this future-proofs the code by precisely conveying the
acceptable registers.  Back-patch to 9.4 (all supported versions).

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20191009063900.GA4066266@rfd.leadboat.com

src/include/storage/s_lock.h

@@ -452,6 +452,11 @@ typedef unsigned int slock_t;
 #define TAS_SPIN(lock)	(*(lock) ? 1 : TAS(lock))
 
 /*
+ * The second operand of addi can hold a constant zero or a register number,
+ * hence constraint "=&b" to avoid allocating r0.  "b" stands for "address
+ * base register"; most operands having this register-or-zero property are
+ * address bases, e.g. the second operand of lwax.
+ *
  * NOTE: per the Enhanced PowerPC Architecture manual, v1.0 dated 7-May-2002,
  * an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.
  * On newer machines, we can use lwsync instead for better performance.
@@ -488,7 +493,7 @@ tas(volatile slock_t *lock)
 #endif
 "	li      %1,0		\n"
 
-:	"=&r"(_t), "=r"(_res), "+m"(*lock)
+:	"=&b"(_t), "=r"(_res), "+m"(*lock)
 :	"r"(lock)
 :	"memory", "cc");
 	return _res;