Provide fallback definitions for SHM_EXEC constant

Although SHM_EXEC is available in <linux/shm.h> since kernel commit
v2.6.9-rc2~23^2~93^2~78, the glibc version of <sys/shm.h> did not
provide SHM_EXEC until commit glibc-2.10~238.

* xlat/shm_flags.in (SHM_EXEC): Add fallback definition.
* tests/shmxt.c (SHM_EXEC): Likewise.

tests/shmxt.c

@@ -18,6 +18,10 @@ cleanup(void)
 # define SHMAT "shmat"
 #endif
 
+#ifndef SHM_EXEC
+# define SHM_EXEC 0100000
+#endif
+
 int
 main(void)
 {

xlat/shm_flags.in

@@ -1,4 +1,4 @@
 SHM_RDONLY
 SHM_RND
 SHM_REMAP
-SHM_EXEC
+SHM_EXEC	0100000