[ELF] Don't reclaim .ctors/.dtors sections.

In FreeBSD, rtld expects .ctors containing -1 (0xffffffff), and a
.ctors section containing the correct bits is provided to the linker as
input (/usr/lib/crtbegin.o).

Contents of section .ctors:
 0000 ffffffff ffffffff                    ........

This section is not stripped even if not referenced or empty, also in
gold or ld.bfd. It would be nice to strip it when not needed but
since existing object files rely on that we can't do better to keep it
around.

Differential Revision:   http://reviews.llvm.org/D15767

llvm-svn: 256373