I was going for a low-hanging fruit example without resorting to bit-twiddling which might confuse many readers, but since you insist I'll repeat my point that going for plain C is great. And that compilers, if properly guided, can optimize better than humans. Following code generates essentially the same code as the GCC builtin intrinsic.

  uint64_t reverse_actually_fast_c(const uint64_t n, const uint64_t k) {
          uint64_t r = 0;
          r = ((n & 0xFF00000000000000) >> 56) |
              ((n & 0x00FF000000000000) >> 40) |
              ((n & 0x0000FF0000000000) >> 24) |
              ((n & 0x000000FF00000000) >>  8) |
              ((n & 0x00000000FF000000) <<  8) |
              ((n & 0x0000000000FF0000) << 24) |
              ((n & 0x000000000000FF00) << 40) |
              ((n & 0x00000000000000FF) << 56);
          r = (r & 0xF0F0F0F0F0F0F0F0) >> 4 | (r & 0x0F0F0F0F0F0F0F0F) << 4;
          r = (r & 0xCCCCCCCCCCCCCCCC) >> 2 | (r & 0x3333333333333333) << 2;
          r = (r & 0xAAAAAAAAAAAAAAAA) >> 1 | (r & 0x5555555555555555) << 1;
          return r >> (64 - k);
      }