Removed linking functionality from gccld.cpp and moved it to linker.cpp.

Renamed functions that were all lower-case.
Moved functions from util.cpp into linker.cpp or gccld.cpp.
Removed util.h and created gccld.h.
Refactored the linker functionality in linker.cpp so that it is easier to
follow, easier to modify, and it's library/object file search behavior is
easier to understand and document.
Added code to include library paths when doing native linking, but this
causes problems and is currently #ifdef'd out.

llvm-svn: 8609