Backport atomic job locking from ksh 93v- beta

Something similar was previously done in 07cc71b8 from a Debian
patch, and eventually reverted; it redefined the ast atomic
functions asoincint() and asodecint() to be gcc-specific. This
imports the upstream version from the ksh 93v- beta instead.

This commit is based on an OpenSUSE patch:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-joblock.dif

src/cmd/ksh93/include/jobs.h:
- Replace job locking mechanism with the 93v- version which uses
  the atomic libast functions asoincint(), asogetint() and
  asodecint(). See: src/lib/libast/man/aso.3

src/cmd/ksh93/sh/jobs.c: job_subsave():
- Revert gcc optimiser bug workaround from c258a04f.
  It should now be unnecessary.

src/cmd/ksh93/include/jobs.h

@@ -33,6 +33,7 @@
 #   include	<signal.h>
 #endif /* !SIGINT */
 #include	"FEATURE/options"
+#include	<aso.h>
 
 #undef JOBS
 #if defined(SIGCLD) && !defined(SIGCHLD)
@@ -125,16 +126,13 @@ extern struct jobs job;
 #define vmbusy()	0
 #endif
 
-#define job_lock()	(job.in_critical++)
+#define job_lock()	asoincint(&job.in_critical)
 #define job_unlock()	\
 	do { \
-		int	sig; \
-		if (!--job.in_critical && (sig = job.savesig)) \
-		{ \
-			if (!job.in_critical++ && !vmbusy()) \
-				job_reap(sig); \
-			job.in_critical--; \
-		} \
+		int	_sig; \
+		if (asogetint(&job.in_critical) == 1 && (_sig = job.savesig) && !vmbusy()) \
+		    job_reap(_sig); \
+		asodecint(&job.in_critical); \
 	} while(0)
 
 extern const char	e_jobusage[];

src/cmd/ksh93/sh/jobs.c

@@ -1882,14 +1882,8 @@ again:
 
 void *job_subsave(void)
 {
-	/*
-	 * We must make a lock first before doing anything else,
-	 * otherwise GCC will remove the job locking mechanism
-	 * as a result of compiler optimization.
-	 */
-	job_lock();
-
 	struct back_save *bp = new_of(struct back_save,0);
+	job_lock();
 	*bp = bck;
 	bp->prev = bck.prev;
 	bck.count = 0;