lib/libc/amd64/string: implement strncat() by calling strlen(), memccpy()

This picks up the accelerated implementation of memccpy().

Tested by:	developers@, exp-run
Approved by:	mjg
MFC after:	1 month
MFC to:		stable/14
PR:		275785
Differential Revision: https://reviews.freebsd.org/D42902

(cherry picked from commit ea7b13771cc9d45bf1bc6c6edad8d1b7bce12990)

lib/libc/amd64/string/Makefile.inc

@@ -17,6 +17,7 @@ MDSRCS+= \
 	strlcat.c \
 	strlcpy.S \
 	strlen.S \
+	strncat.c \
 	strncmp.S \
 	strncpy.c \
 	strnlen.c \

lib/libc/amd64/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);
+}