Update licensing to MIT AND (MIT OR Apache-2.0)

Currently both Cargo.toml and the license files indicate that this
library may be used under either MIT or Apache-2.0. However, this is not
accurate; since portions of this library were derived from musl libc,
which is available under the MIT license, this terms of use for this
library must also include use under the MIT license. That is, it is not
correct that this library may be used under only the Apache-2.0 license.

Update the SPDX license identifier to `MIT OR (MIT AND Apache-2.0)` to
indicate that use must include the MIT license, but to clarify that
contributions are made under `MIT OR Apache-2.0`. This is compatible
with the current state of this repository since it has always contained
both license files, and the `Cargo.toml` license field has indicated
`MIT OR Apache-2.0` since it was added.

In accordance with the above, replace the two license files with a
combined LICENSE.txt that makes these terms clear and gives attribution
to works from which this library is derived.

Link: https://rust-lang.zulipchat.com/#narrow/channel/335408-foundation/topic/Request.20for.20legal.20team.20input.20on.20crate.20licensing

Cargo.toml

@@ -4,7 +4,7 @@ categories = ["no-std"]
 description = "libm in pure Rust"
 documentation = "https://docs.rs/libm"
 keywords = ["libm", "math"]
-license = "MIT OR Apache-2.0"
+license = "MIT AND (MIT OR Apache-2.0)"
 name = "libm"
 readme = "README.md"
 repository = "https://github.com/rust-lang/libm"