std.elf: stronger typing

[dotcarmen]

supercedes #23178

This PR attempts to make std.elf a bit easier for API consumers:

1. many constants have instead been converted to bitfields or enums where compatible and as appropriate
2. ELF32_* and ELF64_* types have been moved/renamed into elf32 and elf64 namespaces
3. all pub const x = switch (@sizeOf(usize)) { ... }; declarations have been consolidated into a single pub const native_elf which selects either elf32 or elf64

note: to optimize the reviewer's experience, i'll keep this PR as 2 commits: the first commit will be the API changes to std.elf, and the 2nd commit will be downstream changes

[dotcarmen]

yes i know i still have to update references elsewhere in the codebase :)

2 questions i have:

1. is it preferred to rename enum fields to lower case?
2. is it preferred to have more descriptive names for the various types? (eg ET -> FileType)

[dotcarmen]

also note that i didn't convert all groups of values to enums/bitfields - for example, AT_ constants can be compared against either u32 or u64 values. if it's preferred that these are also type-ified, I'd be happy to include that in this pr