x86-64: Allocate input section memory if needed

When --no-keep-memory is used, the input section memory may not be cached.
Allocate input section memory for -z pack-relative-relocs if needed.

bfd/

	PR ld/29939
	* elfxx-x86.c (elf_x86_size_or_finish_relative_reloc): Allocate
	input section memory if needed.

ld/

	PR ld/29939
	* testsuite/ld-elf/dt-relr-2i.d: New test.

(cherry picked from commit 9eb71a53)