dtrace: accomodate changes in the 4.1 kernel for sparc64

dtrace: accomodate changes in the 4.1 kernel for sparc64

The handling of some system calls changed:

- The sigaltstack system call is no longer a special case using a
  stub in assembler.
- New system call: execveat

This commit also fixes a problem with the support for the execveat
system call on x86_64, where not all arguments were being passed in
the entry probe for that new system call.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Acked-by: Nick Alcock <nick.alcock@oracle.com>