target: introduce language specific linker flags

This extends the current variable set (`CMAKE_EXE_LINKER_FLAGS`,
`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.
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-minoc-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2019

passed ghostflow-check-master

passed cmake-dash3win10-windows-release-ninja

passed cmake-dash3win10-windows-release-ninja

00:19:54

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:45:34

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:12:45

passed cmake-minoc-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

00:10:29

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

00:11:32

passed cmake-terfin-windows-release-vs2019

passed cmake-terfin-windows-release-vs2019

00:52:13