toolchainfile.cmake.in: set linker flags

The linker flags are part of the toolchain configuration, so set them for
the CMake-based packages.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>