[compiler-rt][cmake] filter libc-backed builtins superseded by assembly (#213481)

apply
https://github.com/llvm/llvm-project/pull/209900#discussion_r3615454574

fix the duplication that causes issues like this:
```
 lit-23: /__w/llvm-project/llvm-project/compiler-rt/test/builtins/Unit/lit.cfg.py:186: fatal: builtins_source_features contains duplicates: ['librt_has_floatdidf', 'librt_has_floatdisf', 'librt_has_floatundidf', 'librt_has_floatundisf']
FAILED: compiler-rt/test/builtins/CMakeFiles/check-builtins /__w/llvm-project/llvm-project/build/compiler-rt/test/builtins/CMakeFiles/check-builtins
cd /__w/llvm-project/llvm-project/build/compiler-rt/test/builtins && /usr/bin/python3 /usr/bin/lit-23 -sv --show-xfail --show-unsupported /__w/llvm-project/llvm-project/build/compiler-rt/test/builtins/TestCases /__w/llvm-project/llvm-project/build/compiler-rt/test/builtins/Unit/X86_64LinuxConfig
ninja: build stopped: subcommand failed.
```

Part of #197824

GitOrigin-RevId: af2a0e9c8897060b4442e7d096459aad922f37d6