Analysis of linkage.c failure on mips-sgi-irix6.5/-mabi=64

[Billinghurst, David (CRTS)]

linkage.c is the last gcc test that passes on irix6.5 with -mabi=n32 and
fails -mabi=64.  The failure is a testsuite problem.

We compile:
* linkage-x.c with gcc -mabi=64
* linkage-y.c with cc (and default to -n32)
* try, and fail, to link these incompatible object files.

This should be XFAILed for mips-sgi-irix6.5/-mabi=64 on the branch.   This
may be best in the long term as well, as I am not sure it is worth fixing.