dropping common asm functions

[Maxime Villard <max%m00nbsd.net@localhost>]

I'd like to remove all the str*.S files from libc/arch/x86_64/string/, and
change the Makefile to use the C version of the functions instead.

Asm doesn't receive instrumentation, and we have to resort to annoying wrappers
all the time (kmsan_strcat, kasan_strcat, etc).

Not relevant for performance, and these functions basically got replaced by
safer variants implemented in C anyway.

Maxime