[Libomptarget] Remove unnecessary CMake definition of endiannness (#77205)

Summary:
This is needed for some definition in `hsa.h` that requires this to be
set for some architectures when it fails at autodetection. We only
really build `libomptarget` with `gcc` and `clang` which already provide
their own way of detecting this. Remove the unnecessary define and move
it into the source.