Introduce a Cargo feature to enable or disable architecture-specific
features (SIMD, assembly), which is on by default. This allows for more
fine grained control compared to relying on the `force-soft-floats`
feature.
Similar to "unstable-intrinsics", introduce a build.rs config option for
`unstable-intrinsics AND NOT force-soft-floats`, which makes this easier
to work with in code.
Effectively, this allows moving our non-additive Cargo feature
(force-soft-floats) to a positive one by default, allowing for an
override when needed.
We currently have a non-additive feature, "force-soft-floats", and we
will need to gain another "no-f16-f128". This makes `cfg` usage in code
somewhat confusing and redundant.
Use `build.rs` to figure out if "unstable-intrinsics" is enabled while
"force-soft-floats" is not enabled and if so, emit a cfg
`intrinsics_enabled`. This is cleaner to use and should make adding more
features easier to reason about.
Also use this as an opportunity to eliminate the build.rs from the
compiler-builtins test crate, replaced with the `[lints]` table in
Cargo.toml.
There isn't any reason for this feature to be exposed or part of the
build script. Move it to a separate crate.
We will also want more tests that require some support functions; this
will create a place for them.
