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In particular, when building dph with the stage2 compiler, we want to
register it in the main package.conf.

The API is the same (for now).  The new implementation has the
capability to define signal handlers that have access to the siginfo
of the signal (#592), but this functionality is not exposed in this
patch.

#2451 is the ticket for the new API.

The main purpose of bringing this in now is to fix race conditions in
the old signal handling code (#2858).  Later we can enable the new
API in the HEAD.

Implementation differences:

 - More of the signal-handling is moved into Haskell.  We store the
   table of signal handlers in an MVar, rather than having a table of
   StablePtrs in the RTS.

 - In the threaded RTS, the siginfo of the signal is passed down the
   pipe to the IO manager thread, which manages the business of
   starting up new signal handler threads.  In the non-threaded RTS,
   the siginfo of caught signals is stored in the RTS, and the
   scheduler starts new signal handler threads.

  - nativeGen/Instruction defines a type class for a generic
    instruction set. Each of the instruction sets we have, 
    X86, PPC and SPARC are instances of it.
  
  - The register alloctors use this type class when they need
    info about a certain register or instruction, such as
    regUsage, mkSpillInstr, mkJumpInstr, patchRegs..
  
  - nativeGen/Platform defines some data types enumerating
    the architectures and operating systems supported by the 
    native code generator.
  
  - DynFlags now keeps track of the current build platform, and 
    the PositionIndependentCode module uses this to decide what
    to do instead of relying of #ifdefs.
  
  - It's not totally retargetable yet. Some info info about the
    build target is still hardwired, but I've tried to contain
    most of it to a single module, TargetRegs.
  
  - Moved the SPILL and RELOAD instructions into LiveInstr.
  
  - Reg and RegClass now have their own modules, and are shared
    across all architectures.

  - There are still some #ifdefs for choosing between i386, x86_64,
      linux, darwin and other OS's.
  - Also reformat SPARC.RegInfo to remove some of the visual noise.

This should fix the build with GHC 6.6

  Darwin 9.6.0 + GCC 4.0.1 doesn't understand "msync". 
  I think "sync" means the same thing.

The function FunDeps.grow was not doing the right thing when type equality
constraints were involved.  That wasn't really its fault: its input was
being filtered by fdPredsOfInsts.

To fix this I did a bit of refactoring, so that the (revolting) fdPredsOfInsts
is now less important (maybe we can get rid of it in due course).  The 'grow'
function moves from FunDeps to
	 Inst.growInstsTyVars
	 TcMTType.growThetaTyVars
	 TcMType.growTyVars

The main comments are with the first of these, in
Note [Growing the tau-tvs using constraints] in Inst.

Push to the branch if conflict free.

We were zeroing the wrong memory

It was getting confused by lines like:
 # 9 "C:\Temp\/ghc620_0/ghc620_0.hc" 1