cmLinkLineComputer: Extract from cmLocalGenerator

CMake has several classes which have too many responsibilities.
cmLocalGenerator is one of them.  Start to extract the link line
computation.  Create generator-specific implementations of the interface
to account for generator-specific behavior.

Unfortunately MSVC60 has different behavior to everything else and CMake
still generates makefiles for it.  Isolate it with MSVC60-specific
names.