GDB on AIX

Hi David and Sergio,

GDB mainline is broken on aix (gcc119.fsffrance.org),

$ ./gdb
GNU gdb (GDB) 7.12.50.20161114-git
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and "show warranty" for details.
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(gdb) kill
terminate called after throwing an instance of 'gdb_exception_RETURN_MASK_ERROR'
IOT/Abort trap (core dumped)

Although we have an aix buildslave on this machine,
I don't think anyone checked the test result carefully.
Here is the first build on this buildslave (on Aug)
http://gdb-build.sergiodj.net/builders/AIX-POWER8-plain/builds/1/steps/regressions/logs/stdio
There are a lot of "UNRESOLVED" and "ERROR", and
summary looks wrong as well, which has only 2330 passes,
but 8323 unresolved.

=== gdb Summary ===

# of expected passes 2330
# of unexpected failures 78
# of expected failures 7
# of known failures 1
# of untested testcases 134
# of unresolved testcases 8323
# of unsupported tests 178

I think Pedro fixed this issue
https://sourceware.org/ml/gdb-patches/2016-04/msg00504.html before,
but I only verified the fix on native arm-linux on reviewing Pedro's fix.
I am not sure it is related to aix 7.1 -> aix 7.2 change.

-- 
Yao (齐尧)