Two small cleanups in libc/src/stdfix.
1. `idiv*` in `stdfix.rst` was marked as implemented for all 12 type
variants but only the 8 non-short width variants (ur, r, ulr, lr, uk, k,
ulk, lk) exist in the directory and are built. Updated the table to
match.
2. Removed `bitusk.cpp` which is an orphaned duplicate of `bitsuk.cpp`
with a typo in the file name. It isn't referenced in the CMakeLists.txt
and has no corresponding .h file.
3. Rename the idivfx test header and helpers.
sohail
e390e6b7a6