[testsuite, rfa] Add gthreads dependency to some failing libstdc++ tests

[Sandra Loosemore]

I ran libstdc++ tests on nios2-elf target.  I observed several new tests 
failing with

error: 'mutex' in namespace 'std' does not name a type

The definition of class mutex in include/bits/std_mutex.h is guarded 
with "#ifdef _GLIBCXX_HAS_GTHREADS" so I assume these tests are not 
supposed to work on this target.  This patch adds the equivalent 
dependency to the failing tests.

OK to commit?  (I guess it is possible that this is actually a bug in 
the code instead, and the tests are supposed to pass....)

-Sandra
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