Abort bigendian builds

[ndim]

This tries to kind of fix #1917 by aborting builds for non-littleendian targets.

[ndim]

Test build on and for s390x:

[...]
+ /usr/bin/cmake -S . -B redhat-linux-build -DCMAKE_C_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_CXX_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_Fortran_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DCMAKE_INSTALL_DO_STRIP:BOOL=OFF -DCMAKE_INSTALL_PREFIX:PATH=/usr -DINCLUDE_INSTALL_DIR:PATH=/usr/include -DLIB_INSTALL_DIR:PATH=/usr/lib64 -DSYSCONF_INSTALL_DIR:PATH=/etc -DSHARE_INSTALL_PREFIX:PATH=/usr/share -DLIB_SUFFIX=64 -DBUILD_SHARED_LIBS:BOOL=ON -D CMAKE_INSTALL_SYSCONFDIR:PATH=/etc -D CMAKE_BUILD_TYPE=build_type=RelWithDebInfo -D BUILD_DOC:BOOL=ON -D HAVE_LINUXSPI:BOOL=ON -D HAVE_LINUXGPIO:BOOL=ON -D HAVE_PARPORT:BOOL=ON -D FETCHCONTENT_FULLY_DISCONNECTED:BOOL=ON -D FETCHCONTENT_QUIET:BOOL=OFF -D BUILD_SHARED_LIBS:BOOL=NO -D USE_STATIC_LIBS:BOOL=YES
-- The C compiler identification is GNU 14.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
CMake Error at CMakeLists.txt:69 (message):
  avrdude cannot be built for a Big Endian target at this time
-- Configuring incomplete, errors occurred!
[...]

All the little endian builds from the CI continue to work. So this appears to work, and the remaining thing is commit message, comments, and error messages. Any input on those?

[ndim]

Back to draft PR as long as @dl8dtl #1917 (comment) is looking into making/keeping avrdude bigendian compatible.