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]: rust-lang/compiler-builtins#624

With the `compiler-builtins` update to 0.1.137 [1], we now provide
symbols necessary to work with `f128` everywhere. This means that we are
no longer restricted to 64-bit linux, and can enable tests by default.

There are still a handful of platforms that need to remain disabled
because of bugs. This patch additionally disables the following:

1. MIPS [2]
2. 32-bit x86 [3]

Math support is still off by default since those symbols are not yet
available.

[1]: rust-lang#132433
[2]: llvm/llvm-project#96432
[3]: llvm/llvm-project#77401