[libcxx] Fix the type in __estimate_column_width

It seems that we are using wchar_t in __estimate_column_width and assume that
it is a 32 bit type. However, on AIX 32 the size of wchar_t is only 16 bits.

Changed wchar_t to uint32_t since the variable is being passed to a function
that uses uint32_t anyway.

Reviewed By: hubert.reinterpretcast, daltenty, Mordante, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D119770