Dejagnu tweak:
Check the compiler output in libstdc++-dg-test using
${tool}_check_unsupported_p and mark the output
unsupported accordingly to avoid "relocation truncated"
failures cluttering the test results on aarch64-none-elf
with -mcmodel=tiny. (gcc torture tests already use this
predicate for this reason.)

This is a revision of
https://gcc.gnu.org/ml/libstdc++/2015-01/msg00198.html
which had problems because the unsupported result was
propagated to and handled by dg-runtest differently
than expected.

Is it OK?

libstdc++-v3/Changelog:

2015-10-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* testsuite/lib/libstdc++.exp (libstdc++-dg-test): Check for
	unsupported compiler output.