If CROSSCOMPILING_EMULATOR was set to an empty string, and a test
was generated with the executable as the command, CMake would segfault
upon trying to generate the test file. Fix this.
Fixes: #18819
Add CROSSCOMPILING_EMULATOR target property for executables. This is used by
subsequent patches to run exectuables created for the target system when
crosscompiling. The property is initialized by the
CMAKE_CROSSCOMPILING_EMULATOR variable when defined.
