Don't check undefined symbol for IFUNC reloc

Don't check undefined symbol for IFUNC reloc

Since x86 elf_*_check_relocs is called after all symbols have been
resolved, there is no need to check undefined symbols for relocations
against IFUNC symbols.

bfd/

	* elf32-i386.c (elf_i386_check_relocs): Don't check undefined
	symbols for relocations against IFUNC symbols.
	* elf64-x86-64.c (elf_x86_64_check_relocs): Likewise.

ld/

	* testsuite/ld-i386/i386.exp: Run pr19636-2e-nacl.
	* testsuite/ld-i386/pr19636-2e.d: Skip for NaCl targets.
	Remove .rel.plt section.
	* testsuite/ld-i386/pr19636-2e-nacl.d: New file.