compat/cuda: add __expf() implementation

compat/cuda/cuda_runtime.h

@@ -184,5 +184,6 @@ static inline __device__ double fabs(double a) { return __builtin_fabs(a); }
 
 static inline __device__ float __sinf(float a) { return __nvvm_sin_approx_f(a); }
 static inline __device__ float __cosf(float a) { return __nvvm_cos_approx_f(a); }
+static inline __device__ float __expf(float a) { return __nvvm_ex2_approx_f(a * (float)__builtin_log2(__builtin_exp(1))); }
 
 #endif /* COMPAT_CUDA_CUDA_RUNTIME_H */