Add a table of all the toolchains we have

[Keno]

I think it would be nice to show exactly what OSes/compilers/libcs etc. we support. I considered spending some time on adding this, but
a) I don't know the current state of things
b) I didn't really know how to best present this information, since there's so many variables
c) I didn't trust my CSS skills to actually implement it

As a result, I'm leaving this here as a TODO. I think the first step here is just to figure out how to present the information, so if somebody wants to do a hand-drawn table that somebody else can then turn in HTML, I think that would be fine.

[ViralBShah]

Is there a way to auto-generate this list?

[giordano]

The list of supported platforms is BinaryBuilder.supported_platforms(), however you need to manually list the available compilers, only the versions of GCC and LLVM can be retrieved somewhat automatically (field version of BinaryBuilderBase.available_gcc_builds and available_llvm_builds)

[ViralBShah]

So is the number of supported toolchains = supported_platforms * (available_gcc_builds + available_llvm_builds)?