[MIPS] Use ia64 backtrace.c.

[Joseph S. Myers]

Thanks.  I've committed this patch.  Note that 
<http://sourceware.org/ml/libc-hacker/2011-12/msg00001.html> proposes 
moving ia64 from libc to ports, in which case other architectures will 
become the canonical home of all files from ia64 that are included by any 
other libc architecture - probably x86_64 will become canonical in this 
case (there's also wordexp.c, sigpending.c and sigprocmask.c).  (For 
backtrace.c it might actually be better for the ia64 version to replace 
the debug/backtrace.c version.)

-- 
Joseph S. Myers
joseph@codesourcery.com