[AIX] Define WCHAR_T_TYPE as unsigned int on 64-bit AIX for wchar.c test

The default wchar type on 64-bit AIX is `unsigned int`, so this patch sets `WCHAR_T_TYPE` to `unsigned int` rather than `int`.

This patch follows similar reasoning to D110428 except for 64-bit.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D113428