Fix s_lock.h PPC assembly code to be compatible with native AIX assembler.

On recent AIX it's necessary to configure gcc to use the native assembler
(because the GNU assembler hasn't been updated to handle AIX 6+).  This
caused PG builds to fail with assembler syntax errors, because we'd try
to compile s_lock.h's gcc asm fragment for PPC, and that assembly code
relied on GNU-style local labels.  We can't substitute normal labels
because it would fail in any file containing more than one inlined use of
tas().  Fortunately, that code is stable enough, and the PPC ISA is simple
enough, that it doesn't seem like too much of a maintenance burden to just
hand-code the branch offsets, removing the need for any labels.

Note that the AIX assembler only accepts "$" for the location counter
pseudo-symbol.  The usual GNU convention is "."; but it appears that all
versions of gas for PPC also accept "$", so in theory this patch will not
break any other PPC platforms.

This has been reported by a few people, but Steve Underwood gets the credit
for being the first to pursue the problem far enough to understand why it
was failing.  Thanks also to Noah Misch for additional testing.

src/include/storage/s_lock.h

@@ -447,6 +447,12 @@ typedef unsigned int slock_t;
  * 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.
+ *
+ * Ordinarily, we'd code the branches here using GNU-style local symbols, that
+ * is "1f" referencing "1:" and so on.  But some people run gcc on AIX with
+ * IBM's assembler as backend, and IBM's assembler doesn't do local symbols.
+ * So hand-code the branch offsets; fortunately, all PPC instructions are
+ * exactly 4 bytes each, so it's not too hard to count.
  */
 static __inline__ int
 tas(volatile slock_t *lock)
@@ -461,20 +467,18 @@ tas(volatile slock_t *lock)
 "	lwarx   %0,0,%3		\n"
 #endif
 "	cmpwi   %0,0		\n"
-"	bne     1f			\n"
+"	bne     $+16		\n"		/* branch to li %1,1 */
 "	addi    %0,%0,1		\n"
 "	stwcx.  %0,0,%3		\n"
-"	beq     2f         	\n"
-"1:	li      %1,1		\n"
-"	b		3f			\n"
-"2:						\n"
+"	beq     $+12		\n"		/* branch to lwsync/isync */
+"	li      %1,1		\n"
+"	b       $+12		\n"		/* branch to end of asm sequence */
 #ifdef USE_PPC_LWSYNC
 "	lwsync				\n"
 #else
 "	isync				\n"
 #endif
 "	li      %1,0		\n"
-"3:						\n"
 
 :	"=&r"(_t), "=r"(_res), "+m"(*lock)
 :	"r"(lock)