Since undefined IFUNC symbol is treated as normal FUNC symbol, don't
abort on undefined IFUNC symbol in the second PLT.
bfd/
PR ld/21401
* elf64-x86-64.c (elf_x86_64_finish_dynamic_symbol): Don't abort
on on undefined IFUNC symbol in the second PLT.
ld/
PR ld/21401
* testsuite/ld-ifunc/ifunc.exp: Add a libtest-2-now.so test with
-z now.