STACKALIGN: Reimplement in terms of __align_down

This changes STACKALIGN to be type-preserving when operating on
pointers.

Reviewed by:	brooks, kib
Effort:		CHERI upstreaming
Sponsored by:	AFRL, DARPA
Differential Revision:	https://reviews.freebsd.org/D54920

sys/arm64/arm64/exec_machdep.c

@@ -838,7 +838,7 @@ sendsig(sig_t catcher, ksiginfo_t *ksi, sigset_t *mask)
 	/* Make room, keeping the stack aligned */
 	fp = (struct sigframe *)addr;
 	fp--;
-	fp = (struct sigframe *)STACKALIGN(fp);
+	fp = STACKALIGN(fp);
 
 	/* Copy the sigframe out to the user's stack. */
 	if (copyout(&frame, fp, sizeof(*fp)) != 0) {

sys/arm64/arm64/vm_machdep.c

@@ -100,7 +100,7 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
 
 	ptrauth_fork(td2, td1);
 
-	tf = (struct trapframe *)STACKALIGN((struct trapframe *)pcb2 - 1);
+	tf = STACKALIGN((struct trapframe *)pcb2 - 1);
 	bcopy(td1->td_frame, tf, sizeof(*tf));
 	tf->tf_x[0] = 0;
 	tf->tf_x[1] = 0;

sys/arm64/include/param.h

@@ -41,7 +41,6 @@
 #include <sys/_align.h>
 
 #define	STACKALIGNBYTES	(16 - 1)
-#define	STACKALIGN(p)	((uint64_t)(p) & ~STACKALIGNBYTES)
 
 #ifndef MACHINE
 #define	MACHINE		"arm64"

sys/arm64/linux/linux_sysvec.c

@@ -338,7 +338,7 @@ linux_rt_sendsig(sig_t catcher, ksiginfo_t *ksi, sigset_t *mask)
 
 	/* Make room, keeping the stack aligned */
 	fp--;
-	fp = (struct l_sigframe *)STACKALIGN(fp);
+	fp = STACKALIGN(fp);
 
 	get_mcontext(td, &uc.uc_mcontext, 0);
 	uc.uc_sigmask = *mask;