This is an unsupported use of the DllImport attribute. Native functions called through DllImport need to be compatible with .NET Platform Invoke(https://learn.microsoft.com/en-us/dotnet/standard/native-interop/pinvoke).
Builtin compiler functions such as __builtin_expect, __builtin_clz, or __builtin_ctz cannot be called through function pointers/pinvoke but must be called directly by C/C++ code. Even if this was supported they wouldn't have the expected effect as the compiler uses them as hints to optimize code. This isn't possible when the function call crosses the managed/unmanaged code barrier.
As a workaround I would suggest moving all the C# functions(PutLE16, PutLE24, PutLE32, BitsLog2Floor, BitsCtz) that use the __builtin_* functions to a C or C++ file in "Assets/Plugins/WebGL" and instead call these functions from the C# code using the DllImport attribute.