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

This patch requires D46243 as it depends on strlcpy being labeled
__strlcpy.

It's a direct copy from the amd64 string functions using memchr and
strlcpy to implement strlcat.

Tested by:	fuz (exprun)
Reviewed by:	fuz, emaste
Sponsored by:	Google LLC (GSoC 2024)
PR:		281175
Differential Revision: https://reviews.freebsd.org/D46272

lib/libc/aarch64/string/Makefile.inc

@@ -29,7 +29,8 @@ MDSRCS+= \
 	strlcpy.S \
 	strncmp.S \
 	memccpy.S \
-	strncat.c
+	strncat.c \
+	strlcat.c
 
 #
 # Add the above functions. Generate an asm file that includes the needed

lib/libc/aarch64/string/memchr.S

@@ -0,0 +1,4 @@
+	.weak memchr
+	.set memchr, __memchr_aarch64
+
+#include "aarch64/memchr.S"

lib/libc/aarch64/string/strlcat.c

@@ -0,0 +1,25 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <sys/cdefs.h>
+
+#include <string.h>
+
+void *__memchr_aarch64(const void *, int, size_t);
+size_t __strlcpy(char *restrict, const char *restrict, size_t);
+
+size_t
+strlcat(char *restrict dst, const char *restrict src, size_t dstsize)
+{
+	char *loc = __memchr_aarch64(dst, '\0', dstsize);
+
+	if (loc != NULL) {
+		size_t dstlen = (size_t)(loc - dst);
+
+		return (dstlen + __strlcpy(loc, src, dstsize - dstlen));
+	} else
+		return (dstsize + strlen(src));
+}