Replacing syscall(_NR_fork) on linux with syscall(_NR_clone)

[David Holmes]

In os::fork_and_exec we provide a per platform fork/exec implementation 
for use by the OnError and OnOutOfMemory JVM options.

On linux this is implemented with direct syscalls to _NR_fork (sys_fork) 
and _NR_execve.

We already encountered a problem on linux-sparc:

http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2015-February/013811.html

due to a different register usage for the return value, which is so far 
restricted to the 7u train but will need addressing in 8u and 9.

Further it seems that the fork syscall has actually been deprecated for 
quite some time and we are now seeing kernels on some platforms which 
are no longer providing any of the deprecated syscalls - we simply get 
an error ENOSYS.

The fork() library was updated to use clone a long time ago, but as I 
understand it we can't use the fork() library call because we might be 
executing in a signal-handling context within the error handler.

So I'm considering switching to use the clone syscall for all linux systems.

Thoughts/comments?

David