x86-64: Handle undefined IFUNC symbol with -z now

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.