cmake: use the toolchain path, id and version as extended signature.

Fixes: #30531

Now using MD5 sum of CMAKE_C_COMPILER (path), CMAKE_C_COMPILER_ID and
CMAKE_C_COMPILER_VERSION variables as part of the TOOLCHAIN_SIGNATURE.

Moved MD5 calculation of executable into
cmake/target_toolchain_flags.cmake to have common location, as
CMAKE_C_COMPILER_ID and CMAKE_C_COMPILER_VERSION are not available until
project() has executed.

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