refactor and simplify ELF loader in objcodes.c

refactor and simplify ELF loader in objcodes.c

* libguile/objcodes.c (sniff_elf_alignment, alloc_aligned)
  (copy_and_align_elf_data): New helpers for portably re-aligning ELF
  data from read(2) or from a bytevector.
  (load_thunk_from_memory): Simplify!  Now there is only one procedure
  that loads ELF, and it does less: it simply receives the whole image
  in one array, hopefully from mmap.

  (scm_load_thunk_from_file): Use new map_file_contents helper, and go
  through load_thunk_from_memory.
  (scm_load_thunk_from_memory): Pass load_thunk_from_memory a piece of
  memory that it owns, and that is appropriately aligned.