build.rs: Cleanup and refactor (#545)

This does some general cleanup and refactoring of `build.rs`. Some as
simple such as putting everything in `mod asm` so we don't need
`#[cfg(feature = "asm")]` everywhere. I also changed the style to more
closely match `meson.build` (e.x. `Define::bool` and `Define::new`
corresponding to `.set` and `set10` vs. just writing out the whole
defines every time. It also unifies the `fn build_nasm_files` and `fn
build_asm_files` to one `fn main` with target checks throughout (again,
more similar to `meson.build`). This PR also keeps the same
output/behavior as before, though I did notice some divergences from
`meson.build` that I'll fix in the next PR.