[m68k] Correct ISAC support

[Richard Sandiford]

Nathan Sidwell <nathan@codesourcery.com> writes:
> 	* elf32-m68k.c (ISAC_PLT_ENTRY_SIZE, elf_isac_plt0_entry,
> 	elf_isac_plt_entry, elf_isac_plt_info): New.

Please add a test for this new PLT layout.  C.f. ld-m68k/plt1-68020.d,
ld-m68k/plt1-isab.d, etc.

Richard