lib/libc/aarch64/string: add strncat SIMD implementation

This patch requires D46170 as it depends on strlcpy being labeled
__memccpy.

It's a direct copy from the amd64 string functions.

Tested by:	fuz (exprun)
Reviewed by:	fuz, emaste
Sponsored by:	Google LLC (GSoC 2024)
PR:		281175
Differential Revision: https://reviews.freebsd.org/D46292

lib/libc/aarch64/string/Makefile.inc

@@ -28,7 +28,8 @@ MDSRCS+= \
 	strcat.c \
 	strlcpy.S \
 	strncmp.S \
-	memccpy.S
+	memccpy.S \
+	strncat.c
 
 #
 # Add the above functions. Generate an asm file that includes the needed

lib/libc/aarch64/string/strncat.c

@@ -0,0 +1,29 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <sys/cdefs.h>
+
+#include <string.h>
+
+void *__memccpy(void *restrict, const void *restrict, int, size_t);
+
+char *
+strncat(char *dest, const char *src, size_t n)
+{
+	size_t len;
+	char *endptr;
+
+	len = strlen(dest);
+	endptr = __memccpy(dest + len, src, '\0', n);
+
+	/* avoid an extra branch */
+	if (endptr == NULL)
+		endptr = dest + len + n + 1;
+
+	endptr[-1] = '\0';
+
+	return (dest);
+}