Keep .plt section and DT_PLTGOT for prelink

Keep .plt section and DT_PLTGOT for prelink

Since the .plt section and DT_PLTGOT are used by prelink to undo
prelinking for dynamic relocations, we must keep them even if there is
no PLT relocation.  This patch reverted commit a3747075a.

bfd/

	* elf32-i386.c (elf_i386_allocate_dynrelocs): Always allocate
	space for the first .plt entry.
	(elf_i386_size_dynamic_sections): Always add DT_PLTGOT for .plt
	section.  Add DT_PLTRELSZ, DT_PLTREL and DT_JMPREL only if
	there are PLT relocations.
	* elf64-x86-64.c (elf_x86_64_allocate_dynrelocs): Always
	allocate space for the first .plt entry.
	(elf_x86_64_size_dynamic_sections): Always add DT_PLTGOT for
	.plt section.  Add DT_PLTRELSZ, DT_PLTREL and DT_JMPREL only if
	there are PLT relocations.

ld/testsuite/

	* ld-i386/i386.exp: Run pltgot-2 for Linux targets.
	* ld-x86-64/x86-64.exp: Likewise.
	* ld-i386/pltgot-1.d: Updated.
	* ld-x86-64/pltgot-1.d: Likewise.
	* ld-i386/pltgot-2.d: New file.
	* ld-x86-64/pltgot-2.d: Likewise.