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At the moment this gives a very slight performance boost of around 1 - 2%.
Future changes to the generated code though so that pointers are kept as
pointers more often instead of being cast to integer types straight away
should hopefully improve the benefit this brings.

Validation fails when validating with PAPI support (i.e. GhcRtsWithPapi  = YES
in validate.mk).  The problem is that the posix symbols are defined by a header
included from papi.h. Compilation then fails because these symbols are
redefined in PosixSource.h. 

This patch adds an undefine for the posix symbols after including papi.h and
before including PosixSource.h. The #undefines are localized to Papi.c since
that is the only case where they are getting defined twice.

 computeLiveness requires the SCCs of blocks to be in reverse dependent
 order, and if they're not it was silently giving bad liveness info, 
 yielding a bad allocation.

 Now it complains, loudly.

 * I've pushed the SPILL and RELOAD instrs down into the
   LiveInstr type to make them easier to work with. 

 * When the graph allocator does a spill cycle it now just
   re-annotates the LiveCmmTops instead of converting them 
   to NatCmmTops and back. 

 * This saves working out the SCCS again, and avoids rewriting
   the SPILL and RELOAD meta instructions into real machine
   instructions.

If there are are unreachable basic blocks in the native code then the 
linear allocator might loop. Detect this case and panic instead.

The problem is that showing SDoc's looks at the static flags global
variables, but those are panics while we are parsing the static flags.
We work around this by explicitly using a fixed prettyprinter style.

Before all the stg registers were simply a bit type or
floating point type but now they can be declared to have
a pointer type to one of these. This will allow various
optimisations in the future in llvm since the type is
more accurate.

These allow annotations of the code produced by the backend
which should bring some perforamnce gains. At the moment
the attributes aren't being used though.

And add a comment about the dangers of int overflow

This patch extends the PAPI support in the RTS to allow collection of native
events. PAPI can collect data for native events that are exposed by the
hardware beyond the PAPI present events. The native events supported on your
hardware can found by using the papi_native_avail tool.

The RTS already allows users to specify PAPI preset events from the command
line. This patch extends that support to allow users to specify native events.
The changes needed are:

1) New option (#) for the RTS PAPI flag for native events. For example, to
   collect the native event 0x40000000, use ./a.out +RTS -a#0x40000000 -sstderr

2) Update the PAPI_FLAGS struct to store whether the user specified event is a
   papi preset or a native event

3) Update init_countable_events function to add the native events after parsing
   the event code and decoding the name using PAPI_event_code_to_name

In the PackageImports extension, import "this" means "import from the
current package".

This phase originally invoked the llvm-as tool that turns a textual
llvm assembly file into a bit code file for the rest of llvm to deal
with. Now the llvm opt tool can do this itself, so we don't need to
use llvm-as anymore.