[libc][stdbit][c23] fixes typos in bit_width, bit_floor C type-generic macros (#84659)

Fixes #84658.

Assuming these were typos in the first place.

I am unsure of the best way to ensure that both sides of the
preprocessor condition in
`libc/include/llvm-libc-macros/stdbit-macros.h` are tested. Could
someone point me in the right direction for adding test coverage to the
non `__cplusplus` branch? Or maybe it is being tested and I've missed
it.