tools/nolibc: Add the definition for dup()

This commit adds the dup() function, which was omitted when sys_dup()
was defined.  This is a port of nolibc's upstream commit 47cc42a79c92
to the Linux kernel.

Fixes: 66b6f755 ("rcutorture: Import a copy of nolibc")
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/nolibc.h

@@ -1852,6 +1852,18 @@ int close(int fd)
 	return ret;
 }
 
+static __attribute__((unused))
+int dup(int fd)
+{
+	int ret = sys_dup(fd);
+
+	if (ret < 0) {
+		SET_ERRNO(-ret);
+		ret = -1;
+	}
+	return ret;
+}
+
 static __attribute__((unused))
 int dup2(int old, int new)
 {