Set CMAKE_EXECUTABLE_SUFFIX to .elf

When linking an executable target, the output binary name CMake passes
to the linker is <target>${CMAKE_EXECUTABLE_SUFFIX}. CMake automatically
sets CMAKE_EXECUTABLE_SUFFIX to .elf for the Arm Compiler but does not
set it for GCC.

We set it in the SDK so that image names do not depend on which compiler
is used, making scripting simpler. The name of an image to use for htrun
is simplified to <target>.${IOTSDK_HTRUN_IMAGE_FORMAT}.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>