Each bit run is upto the right shift count, so the each mask does not
need more upper bits.

Fixes [Bug #17646]

Patch from xtkoba (Tee KOBAYASHI)

These contents are purely implementation details, not worth appearing in
CAPI documents. [ci skip]

As it is an independent feature from `clz`, `ctz` and `popcount`,
it might be unavailable even if the latters are built in.

To fix build failures.

This shall fix compile errors.

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html