[compiler-rt] [compiler-rt][builtins] Fix FLOAT16 feature detection (PR #69842)

[via llvm-commits]

littlewu2508 wrote:

> This fixed a bug in the compiler-rt build configuration that we use in Fedora. I have no idea if this is correct for all build configurations.

I think, unless there's a reason that when `CMAKE_SOURCE_DIR != CMAKE_CURRENT_SOURCE_DIR`, which is the case for Fedora and Gentoo, it (in general) has to be non-static, then there's no reason we should put `STATIC_LIBRARY` inside that `if` branch.

And if there are scenarios `STATIC_LIBRARY` should not be set, then maybe the `if` branch excludes too much cases (Fedora and Gentoo's build configuration), unless our build configuration turned out to be somewhat "incompatible" or "wrong"

https://github.com/llvm/llvm-project/pull/69842