rtld: move relro enforcement after ifunc processing

rtld: move relro enforcement after ifunc processing

Previously the combination of relro (implicit), -z now and ifunc use
resulted in a segfault when applying ifuncs after relro (test binary
here just calls amd64_get_fsbase()):

| % env LD_DEBUG=1 libexec/rtld-elf/obj/ld-elf.so.1 a.out
| ...
| enforcing main obj relro
| ...
| resolving ifuncs
| reloc_jmpslot: *0x203198 = 0x189368ea4570
| zsh: bus error (core dumped)  LD_DEBUG=1 obj/ld-elf.so.1 ~/a.out

Reported by:	Shawn Webb
Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation