exec: Introduce the PROC_PS_STRINGS() macro

Rather than fetching the ps_strings address directly from a process'
sysentvec, use this macro.  With stack address randomization the
ps_strings address is no longer fixed.

Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D33704

sys/arm/arm/exec_machdep.c

@@ -43,6 +43,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/exec.h>
 #include <sys/imgact.h>
 #include <sys/kdb.h>
 #include <sys/kernel.h>
@@ -63,6 +64,11 @@ __FBSDID("$FreeBSD$");
 #include <machine/vfp.h>
 #include <machine/vmparam.h>
 
+#include <vm/vm.h>
+#include <vm/vm_param.h>
+#include <vm/pmap.h>
+#include <vm/vm_map.h>
+
 /*
  * Clear registers on exec
  */
@@ -340,7 +346,7 @@ sendsig(sig_t catcher, ksiginfo_t *ksi, sigset_t *mask)
 	if (sysent->sv_sigcode_base != 0)
 		tf->tf_usr_lr = (register_t)sysent->sv_sigcode_base;
 	else
-		tf->tf_usr_lr = (register_t)(sysent->sv_psstrings -
+		tf->tf_usr_lr = (register_t)(PROC_PS_STRINGS(p) -
 		    *(sysent->sv_szsigcode));
 	/* Set the mode to enter in the signal handler */
 #if __ARM_ARCH >= 7