This patch enhances uprobes to use "single-stepping out of line"
(SSOL) to avoid probe misses in multithreaded applications.  SSOL also
reduces probe overhead by 25-30%.

This patch creates a 1-page VM area in the probed process for SSOL.
This implementation creates the SSOL area only for processes that
are actually probed.  An older implementation, which preallocates
a SSOL area at exec time, can be resurrected if there's interest.
In the current implementation, the size of the SSOL area is fixed
at 1 page.  Slots in the SSOL area are recycled, as necessary, on a
least-recently-used basis.

Comments welcome.

Jim Keniston