[abcdef123xyz123]

In practice - like you say - for non-DSP, non-mainframe this doesn't matter. For applications development, differences between the standard libraries and even compilers are much more difficult than the few differences in architectures. Endiness, strict alignment, char signedness and overflow are about the only things that matter anymore even if you want to support nearly every modern architecture used for popular OSs, and they are relatively easy to handle. Writing C that compiles and works on eg both Unix and Windows is far more about libraries and compiler quirks.