Launchers: Support setting linker launchers

Auxiliary/vim/syntax/cmake.vim

Help/envvar/CMAKE_LANG_LINKER_LAUNCHER.rst

Help/manual/cmake-env-variables.7.rst

Help/manual/cmake-properties.7.rst

Help/manual/cmake-variables.7.rst

Help/prop_tgt/LANG_LINKER_LAUNCHER.rst

Help/release/dev/linker-launcher.rst

Help/variable/CMAKE_LANG_LINKER_LAUNCHER.rst

Modules/CMakeCInformation.cmake

Modules/CMakeCXXInformation.cmake

Modules/CMakeOBJCInformation.cmake

Modules/CMakeOBJCXXInformation.cmake

Source/cmCommonTargetGenerator.cxx

Source/cmCommonTargetGenerator.h

Source/cmMakefileExecutableTargetGenerator.cxx

Source/cmMakefileLibraryTargetGenerator.cxx

Source/cmNinjaNormalTargetGenerator.cxx

Source/cmRulePlaceholderExpander.cxx

Source/cmRulePlaceholderExpander.h

Source/cmTarget.cxx