rtld: remove broken functionality to build exec. version

rtld: remove broken functionality to build exec. version

If anyone had tried to build an executable version of the dynamic linker
using the source makefiles, it would have failed.  The i386 version had
bad CFLAGS and the x86_64 was missing the ldscript.  Moreever, as the
gold linker doesn't use ldscripts, it was GNU linker (ld.bfd) specific.

If somebody decides this functionality is still desired, the ldscript
files from FreeBSD (elf_rtld.x) can be imported from FreeBSD likely
without modification.  The now-dummy makefiles were alterated with
similar comments.