libexec/rtld-elf/rtld-libc/Makefile.inc: do not use machdep string functions

With ifunc based SIMD dispatch, we cannot use the amd64 assembly
implementations of string functions. Modify rtld to instead use
the generic functions. To avoid an architecture-specific special
case, this change is applied to all architectures.

This change is a prerequisite to and formerly part of D40693.

Sponsored by: FreeBSD Foundation
Approved by: kib
See also: D40693
Differential Revision: https://reviews.freebsd.org/D41050