An API for Cache Size(s)

From JuliaLang/julia#6517 - Currently the cache size information is independently hardwired in several methods throughout Julia Base, e.g. in Base.transpose! and in Base.LinAlg.generic_matmatmul, and now also in my implementation of permutedims! (actually in the auxiliary function blockdims). It would probably make sense to centralise this information and maybe even to try to get it from the system (or to set it at build time) instead of fixing it at 32k. - @Jutho

There isn't a new issue to discuss this, but there probably should be. L1 and L2 cache sizes would be nice to have available, but it may require some effort to get this right across OSes and processor types. - @ViralBShah