Put bfd_section_from_shdr loop detection array in elf_tdata

Put bfd_section_from_shdr loop detection array in elf_tdata

The static variables used by bfd_section_from_shdr to detect loops
in ELF sections have a problem:  Comparing a BFD pointer doesn't
guarantee that the current bfd is the same as the one previously used
to allocate the sections_being_created array.  For example, doing
   size bad_elf_1 bad_elf_2
with two corrupted ELF files containing section loops will leave the
section_being_created array allocated for the first file and since
bfd_close is called for bad_elf_1 before bfd_elf_2 is opened, it is
possible that the BFD for the second file is allocated in the same
memory as the first file.  If bad_elf_2 has more sections than
bad_elf_1 then we might write beyond the end of the array.

So this patch implements the FIXME Nick put in a comment about
attaching the array to the BFD.

	* elf-bfd.h (struct elf_obj_tdata): Add being_created.
	* elf.c (bfd_section_from_shdr): Delete static vars for loop
	detection.  Use new tdata variable instead.
	* elfcode.h (elf_object_p): Allocate being_created.