Enable `f128` tests on all non-buggy platforms 🎉

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]: https://github.com/rust-lang/rust/pull/132433
[2]: https://github.com/llvm/llvm-project/issues/96432
[3]: https://github.com/llvm/llvm-project/issues/77401