The first portable programming language was, uh, Fortran. Indeed, by the time the Unix developers are thinking about porting to different platforms, there are already open source Fortran libraries for math routines (the antecedents of LAPACK). And not long afterwards, the developers of those libraries are going to get together and work out the necessary low-level kernel routines to get good performance on the most powerful hardware of the day--i.e., the BLAS interface that is still the foundation of modern HPC software almost 50 years later.

(One of the problems of C is that people have effectively erased pre-C programming languages from history.)