The short answer is 'no'. The longer version is that this primitive (almost certainly) cannot be implemented in macros: primitives rarely can be. In particular,
\pdfstrcmp can do an expandable comparison of two sets of tokens on a character ('string') basis, without loosing any spaces. In the past, the LaTeX3 team did have some code which attempted to do the same thing using only (e-)TeX primitives, but there were limitations and we ended up with issues as a result. The availability of this primitive is very useful for a number of functions which otherwise cannot be implemented expandably, and the team therefore made a decision to require it in addition to those from e-TeX: it's been available for a number of years.