Use correct size for MIPS .rld_map section

On MIPS .dynamic is read-only and so a special section .rld_map is used
to store the pointer to the rtld information for debuggers.  This
section had a hard coded size of 4 bytes which is not correct for
mips64.  (Note that FreeBSD's rtld does not yet populate .rld_map.)

Sponsored by:   DARPA, AFRL
Approved by:	re (delphij)