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As discussed on the libraries/haskell-cafe mailing lists
  http://www.haskell.org/pipermail/libraries/2010-April/013420.html

This is a replacement for block/unblock in the asychronous exceptions
API to fix a problem whereby a function could unblock asynchronous
exceptions even if called within a blocked context.

The new terminology is "mask" rather than "block" (to avoid confusion
due to overloaded meanings of the latter).

In GHC, we changed the names of some primops:

  blockAsyncExceptions#   -> maskAsyncExceptions#
  unblockAsyncExceptions# -> unmaskAsyncExceptions#
  asyncExceptionsBlocked# -> getMaskingState#

and added one new primop:

  maskUninterruptible#

See the accompanying patch to libraries/base for the API changes.

I tried to build ghc-6.12.3 and found out FIW part of code
does not compile anymore. It uses absent functions under #ifdef.
Instead of fixing it I just switched to libffi. Result built successfully
and passed 'foreign import wrapper' test I wrote for trac ticket #3516.

I didn't try to build -HEAD yet, but this patch only removes code, so
it should not make -HEAD worse.

This also means that extsBitmap gets set, whereas is was just being set
to 0 before.

The regular structure type adds padding to conform to the platform ABI,
which causes problems with structures storing doubles under windows since
we don't conform to the platform ABI there. So we use packed structures
instead now that don't do any padding.

This is already handled by the Cmm code generator so LLVM is simply
duplicating work. LLVM also doesn't know which ones are actually live
so saves them all which causes a fair performance overhead for C calls
on x64. We stop llvm saving them across the call by storing undef to
them just before the call.

Patch from Erik de Castro Lopo <erikd@mega-nerd.com>.

'import' syntax is seperate from ':module' syntax

LLVM supports creating pointers in two ways, firstly through
pointer arithmetic (by casting between pointers and ints)
and secondly using the getelementptr instruction. The second way
is preferable as it gives LLVM more information to work with.

This patch changes a lot of pointer related code from the first
method to the getelementptr method.

Instead of calling the C library for these Cmm functions
we use intrinsic functions provided by llvm. LLVM will
then either create a compile time constant if possible, or
use a cpu instruction or as a last resort call the C
library.

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.

 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.