Correct 32-bit architectures for 5.1 and 5.2

[dra27]

Alas, in #24075 I correctly fixed the bytecode-only 64-bit architectures in 5.1 and 5.2 and incorrectly broke the 32-bit architectures.

32-bit architectures can be written as arch = "x86_32" | arch = "arm32" | arch = "ppc32", but I think it's better to put what @Octachron had originally put in the 5.1.0~alpha2 package and have a conjunction of the supported native architectures which is what I've done here (cf. also ocaml/opam#4633).

cc @tmcgilchrist

[tmcgilchrist]

Looks like the right set of options to me.

EDIT The reported arch needs to change after checking on a riscv machine.

[dra27]

Just double-checking the arch value for riscv…

[dra27]

Oops - I took the "riscv" value from #24050, but that's OCaml's architecture value (OCaml has Config.arch = "riscv" and Config.model = "riscv64", differing from uname -m's value)

[avsm]

Thanks! We do need to document the 'useful' arch values somewhere; it's a perpetual source of ambiguity.