[libc++] Remove <stddef.h> includes from the granularized <cstddef> headers (#114788)

We can define some of these aliases without having to include the system
<stddef.h> and there doesn't seem to be much of a reason we shouldn't do
it this way.

GitOrigin-RevId: 5acc4a3dc0e2145d2bfef47f1543bb291c2b866a
12 files changed