Add more platforms to the download page

[alexrp]

Posting this for tracking purposes due to #415 (comment).

Besides s390x-linux, I think we should also add powerpc64-linux (big endian) as there's been a fair amount of interest in that platform on the Zig issue tracker and communities.

The rest of the triples we support are probably too niche to put on the download page (think armeb-linux, aarch64_be-linux, powerpc-linux (32-bit), riscv32-linux, etc), or in the case of MIPS, too much of a horribly fragmented mess where we would need to build a dozen or so binaries to cover all the hardware that actually sees real use.