tzsetup: ask local/UTC question only on x86

Storing local time in the RTC is a legacy of 1990s PCs; it's not
relevant on other platforms of interest to FreeBSD.

While here switch to C99 bool.

Sponsored by: The FreeBSD Foundation
Reviewed by: allanjude (earlier), imp (earlier)
Differential Revision: https://reviews.freebsd.org/D45575