Ensure that STAP_PROBE behaves like a single statement

Ensure that STAP_PROBE behaves like a single statement

It's important for macros like this to appear as a single statement, so
they won't break the control flow of the caller.  The new STAP_SEMAPHORE
macro was breaking this masquerade, but now I've moved it inside of the
do-while block.

* includes/sys/sdt.h (STAP_SEMAPHORE): Define the disabled case empty.
  (STAP_PROBE[1-10]_): Move the semaphore inside the do-while block.