cmake: deprecate toolchain_ld_<base|baremetal|cpp> macros

Remove the toolchain_ld_<base|baremetal|cpp> macro as all the macro
handling is now done through the use of linker properties.

Keep support for calling the old macros for out of tree toolchains
which have not been updated to the new property approach.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>