libmach:

libmach:
	* heuristic to go farther during stack traces.
	* significantly improved Linux thread handing.

acid:
	* update to new libmach interface.

prof:
	* use new libmach interface.
	* multiple thread support (derived from Rob's copy).
	* first steps toward pprof-like graphs:
	  keep counters indexed by pc,callerpc pairs.

R=r
DELTA=909  (576 added, 123 deleted, 210 changed)
OCL=24240
CL=24259