linker: move where we define _LINKER and _ASMLANGUAGE

Move _LINKER and _ASMLANGUAGE to target.cmake because of how we pick the
linker script that might be used.  This way regardless of how or where a
linker.ld gets included we will always set _LINKER & _ASMLANGUAGE (so
any header that needs check based on those defines they can,
specifically generated_dts_board.h)

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>