Make profiling work with multiple capabilities (+RTS -N)

This means that both time and heap profiling work for parallel
programs.  Main internal changes:

  - CCCS is no longer a global variable; it is now another
    pseudo-register in the StgRegTable struct.  Thus every
    Capability has its own CCCS.

  - There is a new built-in CCS called "IDLE", which records ticks for
    Capabilities in the idle state.  If you profile a single-threaded
    program with +RTS -N2, you'll see about 50% of time in "IDLE".

  - There is appropriate locking in rts/Profiling.c to protect the
    shared cost-centre-stack data structures.

This patch does enough to get it working, I have cut one big corner:
the cost-centre-stack data structure is still shared amongst all
Capabilities, which means that multiple Capabilities will race when
updating the "allocations" and "entries" fields of a CCS.  Not only
does this give unpredictable results, but it runs very slowly due to
cache line bouncing.

It is strongly recommended that you use -fno-prof-count-entries to
disable the "entries" count when profiling parallel programs. (I shall
add a note to this effect to the docs).

1 files changed, 3 insertions, 0 deletions

diff --git a/rts/RetainerProfile.h b/rts/RetainerProfile.h
index b2adf71d9d..0e75327cde 100644
--- a/rts/RetainerProfile.h
+++ b/rts/RetainerProfile.h
@@ -21,6 +21,9 @@ void endRetainerProfiling  ( void );
 void retainerProfile       ( void );
 void resetStaticObjectForRetainerProfiling( StgClosure *static_objects );
 
+// flip is either 1 or 0, changed at the beginning of retainerProfile()
+// It is used to tell whether a retainer set has been touched so far
+// during this pass.
 extern StgWord flip;
 
 // extract the retainer set field from c