benchmarks/uica: fix build on riscv and in UTC-... time zones

• inform mbuild CPU type detection script about the existence of this newfangled architecture.
• pass TZ=UTC to date(1) while generating distfile name to avoid off by one error on the first day of the month in time zones west of Greenwich.

PR: 267020
Submitted by: maintainer
MFH: 2022Q4

(cherry picked from commit 56f224add65df796bf57c49ea782e00470a56885)