Fix objtool: missing int3 after ret warning

Resolve straight-line speculation warnings reported by objtool
for x86_64 assembly on Linux when CONFIG_SLS is set.  See the
following LWN article for the complete details.

https://lwn.net/Articles/877845/

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575

module/icp/include/sys/ia32/asm_linkage.h

@@ -30,6 +30,12 @@
 #include <sys/stack.h>
 #include <sys/trap.h>
 
+#if defined(__linux__) && defined(CONFIG_SLS)
+#define	RET	ret; int3
+#else
+#define	RET	ret
+#endif
+
 #ifdef	__cplusplus
 extern "C" {
 #endif