mips/makecontext.c: Use STACK_ALIGNBYTES for clarity.

No functional change intended.

Tidying to make stack alignment more obvious, prompted by:

PR kern/59327: user stack pointer is not aligned properly

Part of a patch by rin@.

Author: riastradh

lib/libc/arch/mips/gen/makecontext.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: makecontext.c,v 1.9 2025/04/24 23:51:43 riastradh Exp $	*/
+/*	$NetBSD: makecontext.c,v 1.10 2025/04/24 23:54:13 riastradh Exp $	*/
 
 /*-
  * Copyright (c) 2001 The NetBSD Foundation, Inc.
@@ -31,9 +31,11 @@
 
 #include <sys/cdefs.h>
 #if defined(LIBC_SCCS) && !defined(lint)
-__RCSID("$NetBSD: makecontext.c,v 1.9 2025/04/24 23:51:43 riastradh Exp $");
+__RCSID("$NetBSD: makecontext.c,v 1.10 2025/04/24 23:54:13 riastradh Exp $");
 #endif
 
+#include <sys/param.h>
+
 #include <inttypes.h>
 #include <stdarg.h>
 #include <stddef.h>
@@ -54,16 +56,13 @@ makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...)
 	/* LINTED uintptr_t is safe */
 	sp  = (__greg_t *)
 	    ((uintptr_t)ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
-	/* LINTED uintptr_t is safe */
 #if defined(__mips_o32) || defined(__mips_o64)
 	sp -= (argc >= 4 ? argc : 4);	/* Make room for >=4 arguments. */
-	sp  = (__greg_t *)
-	      ((uintptr_t)sp & ~0x7);	/* Align on double-word boundary. */
 #elif defined(__mips_n32) || defined(__mips_n64)
 	sp -= (argc > 8 ? argc - 8 : 0); /* Make room for > 8 arguments. */
-	sp  = (__greg_t *)
-	      ((uintptr_t)sp & ~0xf);	/* Align on quad-word boundary. */
 #endif
+	/* LINTED uintptr_t is safe */
+	sp  = (__greg_t *)((uintptr_t)sp & ~STACK_ALIGNBYTES);
 
 	gr[_REG_SP]  = (intptr_t)sp;
 	gr[_REG_RA]  = (intptr_t)__resumecontext;