[DO NOT MERGE] Update libchdr

[negativeExponent]

Update libchdr to latest upstream.

Fix: https://github.com/libretro/beetle-supergrafx-libretro/issues/138

[LibretroAdmin]

I think it'd be better to do this with a git subtree. If you could change it to that, I'd feel more comfortable with merging it.

I'll also have to conduct a test on the buildbot to see if this still builds for all platforms