avoid UB in sinfl_bsr

[RicoP]

sinfl_bsr has undefined behavior for compilers other than MSVC, GCC and clang (in my case Tiny C Compiler) because the function won't return anything at all.

I turned the #elif into a #else so on compilers other than MSVC sinfl_bsr will always rely on __builtin_clz. This can still cause a compile error on compilers that don't support __builtin_clz but at least we have a explicit error. Alternatively I could also implement sinfl_bsr by hand:

static int
sinfl_bsr(unsigned n) {
#ifdef _MSC_VER
  unsigned long r = 0;
  _BitScanReverse(&r, n);
  return int(r);
#elif defined(__GNUC__) || defined(__clang__) || defined(__TINYC__)
  return 31 - __builtin_clz(n);
#else  
    static const unsigned char table_2_32[32] = {
        31, 22, 30, 21, 18, 10, 29,  2, 20, 17, 15, 13,  9,  6, 28,  1,
        23, 19, 11,  3, 16, 14,  7, 24, 12,  4,  8, 25,  5, 26, 27,  0
    };
    n |= n >> 1;  
    n |= n >> 2;  
    n |= n >> 4;  
    n |= n >> 8;  
    n |= n >> 16; 
    n = table_2_32[(n * 0x07c4acddu) >> 27];
    return 31 - n;
#endif
}

I got this code from https://github.com/TinyCC/tinycc/blob/6ec4a106524ab6a6e3a4178774fd1e884d3fcce2/lib/builtin.c#L50