Addition of GDB server monitor command 'v.info execontext' that shows
information about the stack traces recorded by Valgrind.
This can be used to analyse one possible cause of Valgrind high
memory usage for some programs.

At work, a big set of regression tests crashed out of memory under Valgrind.

Two main causes for out of memory were identified:
1. big memory usage for stacktrace (exe contexts) recording by Valgrind
2. big number of partially initialised bytes.

This patch adds a gdbsrv monitor command that output (very) detailed
information about all the recorded exe context.

This has been used to analyse the problem 1. above,
showing the following identified causes for a (too) big nr of execontexts:

A. When the JIT handles an unknown SP update, even when --track-origins=no,
an execontext is (uselessly) created and recorded
to track the (never used) origin of some uninitialised stack memory.
This creates a whole bunch of 'one IP' execontexts.

B. same problem in handling some system calls (at least the brk system
 calls always records an origin, even when --track-origins=yes).

C. The Valgrind unwinder cannot properly unwind some stack traces.
  It unwinds a few frames, then go bezerk and stops at a "random" IP.
  This then causes the same "logical" stacktrace to be truncated
  and records thousands of times with this "differentiating" last IP.


For problem cause 2 above ( a lot of partially initialised bytes),
the idea is to similarly add another gdbsrv commands that will output
statistics about which stack traces are causing a lot of uninitialised bytes. 




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13220 a5019735-40e9-0310-863c-91ae7b9d1cf9