lib/libc/aarch64/string: strcat enable use of SIMD

Call into SIMD strlen and stpcpy for an optimized strcat. Port of
D42600 for amd64.

Tested by:	fuz (exprun)
Reviewed by:	fuz, emaste
Sponsored by:	Google LLC (GSoC 2024)
PR:		281175
Differential Revision: https://reviews.freebsd.org/D46417

lib/libc/aarch64/string/Makefile.inc

@@ -25,7 +25,8 @@ MDSRCS+= \
 	strspn.S \
 	strcspn.S \
 	strpbrk.c \
-	strsep.c
+	strsep.c \
+	strcat.c
 
 #
 # Add the above functions. Generate an asm file that includes the needed

lib/libc/aarch64/string/strcat.c

@@ -0,0 +1,20 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Getz Mikalsen <getz@FreeBSD.org>
+*/
+
+#include <string.h>
+
+#undef strcat	/* _FORTIFY_SOURCE */
+
+char *
+strcat(char * __restrict s, const char * __restrict append)
+{
+	char *save = s;
+
+	/* call into SIMD optimized functions */
+	stpcpy(s + strlen(s), append);
+
+	return(save);
+}