Use the compiler to determine whether or not to enable `f16` and `f128`

Currently we whether or not to build and test `f16` and `f128` support
mostly based on the target triple. This isn't always accurate, however,
since support also varies by backend and the backend version.

Since recently, `rustc` is aware of this with the unstable config option
`target_has_reliable_{f16,f128}`, which better represents when the types
are actually expected to be available and usable. Switch our
compiler-builtins and libm configuration to use this by probing `rustc`
for the target's settings.

A few small `cfg` fixes are needed with this.

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