Include <syms.h>, not <sym.h> on IRIX. Removed

Include <syms.h>, not <sym.h> on IRIX.  Removed
duplicate definition of ElfW.
(find_section): Copied from unexsgi.c.
(unexec): Use find_section.  Adjust whitespace.  Initialize
new_data2_offset based on old_data, not sbss (this fixes a bug on
IRIX6).  Change #ifdef __mips to __sgi, since it's IRIX-specific.
Adjust test for presence of .mdebug section to the new return
value of find_section.

Merge changes from 20.5.
(unexec): Handle .lit4 and .lit8 unconditionally.