Implementation for tracing stub-based system calls. Due to the need for

Implementation for tracing stub-based system calls.  Due to the need for
specialized code handling (mainly passing in a pt_regs structure as one of
the arguments), some syscalls are called through a stub in assembly code.
We duplicate the stub cdode in dtrace_stubs_x86_64.S, but instead of calling
the actual syscall implementation code call our own syscall-specific handler,
which ensures that entry and return probes are called as enabled, and then
call the underlying implementation directly for handling the syscall.

Also removed debugging output that is no longer relevant (code cleanup).

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>