Update `compiler_builtins` to 0.1.138 and pin it

This updates to a new version of builtins that includes [1], which was
the last blocker to us enabling `f128` tests on all platforms 🎉.

With this update, also change to pinning the version with `=` rather
than using the default carat versioning. This is meant to ensure that
`compiler-builtins` does not get updated as part of the weekly
`Cargo.lock` update, since updates to this crate need to be intentional:
changes to rust-lang/rust and rust-lang/compiler-builtins sometimes need
to be kept in lockstep, unlike most dependencies, and sometimes these
updates can be problematic.

[1]: https://github.com/rust-lang/compiler-builtins/pull/624