handle `typemin(::Signed)` inputs more consistently across `gcd`, `gcdx`, `lcm`

[adienes]

continuation of #59628 (comment) ; that PR fixed the most urgent correctness problems but left open a few (existing) warts in consistency across the three functions

I think this is mostly about throwing more sensible errors rather than deeper in the machinery, but there are a few examples of gcdx that are not errors but should. also the rule should be documented ("cannot be typemin signed")

I can try to do this soon but opening the issue so I don't forget

[aryan98795]

looks good can u assign this issue to me

[adienes]

we do not assign issues; you are free to work on anything you like