Add and move fall-through comments in system-specific code.

Add and move fall-through comments in system-specific code.

This patch fixes -Wimplicit-fallthrough warnings in system-specific
code that show up building glibc with -Wextra, by adding fall-through
comments, or moving existing such comments to the place required for
them to work (immediately before the case label being fallen through).

Tested with build-many-glibcs.py.

	* sysdeps/i386/dl-machine.h (elf_machine_rela): Add fall-through
	comments.
	* sysdeps/m68k/m680x0/fpu/s_cexp_template.c (s(__cexp)): Likewise.
	* sysdeps/m68k/memcopy.h (WORD_COPY_FWD): Likewise.
	(WORD_COPY_BWD): Likewise.
	* sysdeps/mach/hurd/ioctl.c (__ioctl): Likewise.
	* sysdeps/powerpc/powerpc64/dl-machine.h (elf_machine_rela):
	Likewise.
	* sysdeps/s390/iso-8859-1_cp037_z900.c (TR_LOOP): Likewise.
	* sysdeps/mips/dl-machine.h (elf_machine_reloc): Move fall-through
	comment.
	* sysdeps/mips/dl-trampoline.c (__dl_runtime_resolve): Likewise.