target: introduce language specific linker flags
This extends the current variable set (
CMAKE_SHARED_LINKER_FLAGS) with language specific linker flags. Doing
this allows for language dependent flags to be set which is required for
mixed-language projects which use different linkers (e.g. Swift which
uses the swift driver and C/C++ which use the cc/c++ driver). Because
this extends the existing variable, no policy should be needed.