Target1

OK, first ice-breaker of a few.  I initially mentioned this here: http://bit.ly/XKFUa4

But to give a rundown of what Target is:

Target is intended to be a data structure that describes a backend target.  The ```target.h``` is a shared structure common to all, leaving each compiler backend left to implement their own ```target.c``` that provides the correct information for the targets they support.

This aims to reduce the need for GDC and LDC to use lots of ```#if IN_GCC``` or ```#if LLVM``` conditions in their own copies that deviate away from the hosted source of the D frontend here on github.

Haven't tested on windows, but I'll see what happens when the auto-tester kicks in.

Feedback welcome.
Thanks,
Iain.