Don't use _pext_u64() directly

Don't use _pext_u64() directly

This intrinsic to call BMI2 PEXT instruction is
defined in immintrin.h. This header should be
included only when USE_PEXT is defined, otherwise
we define _pext_u64 as 0 forcing a nop.

But under some mingw platforms, even if we don't
include the header, immintrin.h gets included
anyhow through an include chain that starts with
STL <algorithm> header. So we end up both defining
_pext_u64 function and at the same time defining
_pext_u64 as 0 leading to a compile error.

The correct solution is of not using _pext_u64 directly.

This patch fixes a compile error with some mingw64
package when compiling with x86-64.

No functional change.

Resolves #222