Linker reorganization

[Ben Gamari]

Hello RTS people,

Today I finally grew frustrated enough with my constant battle with the
7000 line tangle of CPP that is rts/Linker.c to do something about it.
The result is D2643 through D2650. In short, I took the file and chopped
it into more managable pieces:

 * linker/PEi386.[ch]:       PE loading
 * linker/MachO.[ch]:        MachO loading
 * linker/Elf.[ch]:          ELF loading
 * linker/CacheFlush.[ch]:   Platform-specific icache flushing logic
 * linker/SymbolExtras.[ch]: Symbol extras support logic
 * Linker.c:                 Everything necessary to glue all of the above together
 * LinkerInternals.h:        Declarations shared by the above and
                             declarations for Linker.c

For the most part this involved just shuffling code around since there
was some rough platform abstraction already in place. In fact, I tried
quite hard to avoid performing any more intricate refactoring to keep
the scope of the project in check. Consequently, this is only a start
and the design is in places a bit awkward; there is still certainly no
shortage of work remaining to be done. Regardless, I think this change
an improvement over the current state of affairs.

One concern that I have is that the RTS's header file structure (where
everything is #include'd via Rts.h) doesn't work very well for this
particular use, where we have a group of headers specific to a
particular subsystem (e.g. linker/*.h). Consequently, these header files
currently lack enclosing `extern "C"` blocks (as well as
Begin/EndPrivate blocks). It would be easy to add these, but I was
curious to hear if others had any better ideas.

The refactoring was performed over several small-ish commits, so review
shouldn't be so bad. I expect to rebase the LoadArchive.c refactoring
performed in D2642 on top of this set once it has been merged. I will
also offer to rebase DemiMarie's recent error-handling patch (D2652).

I have tested the set on a variety of platforms,

 * x86-64 Linux
 * x86-64 Darwin
 * x86-64 FreeBSD
 * x86-64 Windows
 * ARM Linux

What do you think?

Cheers,

- Ben
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