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

[fuz]

Summary

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

Test Plan

Passed make tinderbox as part of D40693.

[kib]

In the commit message, I would emphasize that the change (partially) fixes existing bug where libc use of non-GPR registers or ifunc could break rtld.

[fuz]

I'm sorry for not taking your note into account when landing the commit. I was preparing the commit as you were writing this.

In the commit message, I would emphasize that the change (partially) fixes existing bug where libc use of non-GPR registers or ifunc could break rtld.

We might want to compile rtld with -mgeneral-regs-only to guarantee that the compiler abstains from use of any SIMD registers. This may be more future-proof than -mno-sse and more portable, too.

[kib]

In D41050#934580, @fuz wrote:

We might want to compile rtld with -mgeneral-regs-only to guarantee that the compiler abstains from use of any SIMD registers. This may be more future-proof than -mno-sse and more portable, too.

Yes we do. Although it perhaps requires per-arch approach.