tools/nolibc: Implement fork() based on clone()

Some archs such as arm64 do not have fork() and have to use clone()
instead.  This commit therefore makes fork() use clone() when
available. This requires including signal.h to get the definition of
SIGCHLD.  This is a port of nolibc's upstream commit d2dc42fd6149 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>