Changes for mmap; details in change log.

Changes for mmap; details in change log.

Added some new interfaces, and a new entry in the target vector.  Under the new
interfaces, mmap will be used if available, otherwise malloc/seek/read, as
before.  Old interfaces all still intact.

Most configurations (including all used by "--enable-targets=all") simply
changed to call the default routine for that entry in the target vector.  I
might've missed some targets only included in special configurations.

Support for a.out symbol and string table reading now goes through new
interfaces, and will use mmap when available.

Linker hooks (e.g., avoiding reallocation under malloc) not ready yet.