PR30326, uninitialised value in objdump compare_relocs

PR30326, uninitialised value in objdump compare_relocs

This is a fuzzing PR, with a testcase involving a SHF_ALLOC and
SHF_COMPRESSED SHT_RELA section, ie. a compressed dynamic reloc
section.  BFD doesn't handle compressed relocation sections, with most
of the code reading relocs using sh_size (often no bfd section is
created) but in the case of SHF_ALLOC dynamic relocs we had some code
using the bfd section size.  This led to a mismatch, sh_size is
compressed, size is uncompressed, and from that some uninitialised
memory.  Consistently using sh_size is enough to fix this PR, but I've
also added tests to exclude SHF_COMPRESSED reloc sections from
consideration.

	PR 30362
	* elf.c (bfd_section_from_shdr): Exclude reloc sections with
	SHF_COMPRESSED flag from normal reloc processing.
	(_bfd_elf_get_dynamic_reloc_upper_bound): Similarly exclude
	SHF_COMPRESSED sections from consideration.  Use sh_size when
	sizing to match slurp_relocs.
	(_bfd_elf_canonicalize_dynamic_reloc): Likewise.
	(_bfd_elf_get_synthetic_symtab): Use NUM_SHDR_ENTRIES to size
	plt relocs.
	* elf32-arm.c (elf32_arm_get_synthetic_symtab): Likewise.
	* elf32-ppc.c (ppc_elf_get_synthetic_symtab): Likewise.
	* elf64-ppc.c (ppc64_elf_get_synthetic_symtab): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_get_synthetic_symtab): Likewise.