localedef: Fix a wchar_t comparison

The intent here is to characterize wide characters which fit in a single
byte. When wchar_t is signed, this comparison is insufficient.

On systems with signed wchar_t, of the files in /usr/share/locale this
only affects zh_CN.GB18030/LC_COLLATE.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50757

(cherry picked from commit a37bc4cb8d2c5c8402341c9df75580f60bb34186)