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: 66b6f755ad ("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

@@ -271,6 +271,8 @@ struct stat {
 #define WEXITSTATUS(status)   (((status) & 0xff00) >> 8)
 #define WIFEXITED(status)     (((status) & 0x7f) == 0)
 
+/* for SIGCHLD */
+#include <asm/signal.h>
 
 /* Below comes the architecture-specific code. For each architecture, we have
  * the syscall declarations and the _start code definition. This is the only
@@ -1529,7 +1531,15 @@ int sys_execve(const char *filename, char *const argv[], char *const envp[])
 static __attribute__((unused))
 pid_t sys_fork(void)
 {
+#ifdef __NR_clone
+	/* note: some archs only have clone() and not fork(). Different archs
+	 * have a different API, but most archs have the flags on first arg and
+	 * will not use the rest with no other flag.
+	 */
+	return my_syscall5(__NR_clone, SIGCHLD, 0, 0, 0, 0);
+#else
 	return my_syscall0(__NR_fork);
+#endif
 }
 
 static __attribute__((unused))