libc: Fix build with WITHOUT_MACHDEP_OPTIMIZATIONS=YES set.

Test Plan: make buildword WITHOUT_MACHDEP_OPTIMIZATIONS=YES on 14-CURRENT and 13-STABLE

Reviewed by: emaste

Differential Revision: https://reviews.freebsd.org/D38017

PR: 266900
(cherry picked from commit 016e46fd869ebf9891ca4b2cf1d22b337717a8c8)