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>