GDB/gcc-6: catch_stack_overflow fails on arm-elf #999

[Joel Brobecker]

See https://gaia.eu.adacore.com/testsuites/report/15032633

The problem appears to be an inability to unwind from
__gnat_debug_raise_exception all the way to the user code
after stack-overflow detection caused a S_E exception.

We should take a look before switching the arm-elf wavefronts
to gcc-6.

assigned to brobecker
copy to hainque, rogers

-- 
Joel