Position Independent Code and Dynamic Linking Support, Part 1

This commit allows generation of position independent code (PIC) that fully supports dynamic linking on Mac OS X and PowerPC Linux.
Other platforms are not yet supported, and there is no support for actually linking or using dynamic libraries - so if you use the -fPIC or -dynamic code generation flags, you have to type your (platform-specific) linker command lines yourself.


nativeGen/PositionIndependentCode.hs:
New file. Look here for some more comments on how this works.

cmm/CLabel.hs:
Add support for DynamicLinkerLabels and PIC base labels - for use inside the NCG.
needsCDecl: Case alternative labels now need C decls, see the codeGen/CgInfoTbls.hs below for details

cmm/Cmm.hs:
Add CmmPicBaseReg (used in NCG),
and CmmLabelDiffOff (used in NCG and for offsets in info tables)

cmm/CmmParse.y:
support offsets in info tables

cmm/PprC.hs:
support CmmLabelDiffOff
Case alternative labels now need C dec...