perf tool: Handle legacy syscalls

Older format does not have the 'nr' field:
    field:int nr;	offset:8;	size:4;	signed:1;

So field processing needs to start with the first one, not 'next'.

This really should be keyed off the individual syscall events, not the use
of syscalls over raw_syscalls. Come back and fix this.

Signed-off-by: David Ahern <dsahern@gmail.com>
perf trace: Improve print of read and write syscalls

Second argument is an address -- either buffer or iov. Print as hex.

Signed-off-by: David Ahern <dsahern@gmail.com>
perf trace: Improve print of open syscalls

First argument is a buffer with a pathname. Print address as hex.

Signed-off-by: David Ahern <dsahern@gmail.com>