I don't understand the question. I mean, is it true that we have a one-to-one (or zero) map from platforms to default standard libraries?
It is set at the time the compiler is built. I wouldn't say it's "platform" specific. As this is chosen by whoever built the compiler (e.g.: the maintainer for the gcc package in Debian).
If it is not true, then I misunderstood it from the beginning. And if it is true, why can't the build system get the maps then get the standard libraries by the current platforms?
That interface doesn't exist in a well defined way today. In large part because it's not necessary, as the build system doesn't quite need to know that the std library even is.
Daniel