All of the backend relocate_section functions that interpret reloc
numbers assuming the input file is of the expected type (ie. same as
output or very similar) really ought to be checking input file type.
Not many do, and those that do currently just assert.  This patch
replaces the assertion with a more graceful exit.

	PR 23980
	* elf32-i386.c (elf_i386_relocate_section): Exit with wrong format
	error rather than asserting input file is as expected.
	* elf32-s390.c (elf_s390_relocate_section): Likewise.
	* elf32-sh.c (sh_elf_relocate_section): Likewise.
	* elf32-xtensa.c (elf_xtensa_relocate_section): Likewise.
	* elf64-ppc.c (ppc64_elf_relocate_section): Likewise.
	* elf64-s390.c (elf_s390_relocate_section): Likewise.
	* elf64-x86-64.c (elf_x86_64_relocate_section): Likewise.
	* elf32-ppc.c (ppc_elf_relocate_section): Exit with wrong format
	error if input file is not ppc32 ELF.