[RFC] zstd as a compression algorithm for LTO

[Martin Liška]

Hello.

As mentioned by Honza, it's using cmake and to be honest I prefer to use a shared
library than a statically build library. Moreover, it's an optional requirement and
so that we don't have to include that to contrib/download_prerequisites.

I like the idea of marking of compression algorithm in 'LTO_header'. However,
we do compress the header as well. Proper solution would be to make a new
section .gnu.lto_.header where we'll put:
struct lto_header
{
  int16_t major_version;
  int16_t minor_version;
};

I don't see a reason why why should have that information in each LTO ELF section?

In the time being, I've written the code so that I fallback in decompression to zlib
if ZSTD detects that LTO bytecode was compressed with zlib. On the contrary, decompression
of zstd with zlib will end with:
lto1: internal compiler error: compressed stream: data error

I'm sending updated version of the patch that can properly detect zstd.

Martin

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