dtrace: stub-based syscall tracing

dtrace: stub-based syscall tracing

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>