[Obvious] Fix libstdc++/33394 testcase when cross-testing linux

[Alan Lawrence]

When cross-testing, the -DITERATIONS=1000 flag replaced the -pthread required 
for linux targets, so the test failed to build. I've pushed the following test 
fix as r221666:

Index: libstdc++-v3/testsuite/21_strings/basic_string/pthread33394.cc
===================================================================
--- libstdc++-v3/testsuite/21_strings/basic_string/pthread33394.cc 
(revision 221665)
+++ libstdc++-v3/testsuite/21_strings/basic_string/pthread33394.cc      (working 
copy)
@@ -18,7 +18,7 @@
  // { dg-do run { target *-*-freebsd* *-*-dragonfly* *-*-netbsd* *-*-linux* 
*-*-gnu* *-*-solaris* *-*-cygwin *-*-darwin* } }
  // { dg-options "-pthread" { target *-*-freebsd* *-*-dragonfly* *-*-netbsd* 
*-*-linux* *-*-gnu* *-*-solaris* } }

-// { dg-options "-DITERATIONS=1000" { target simulator } }
+// { dg-additional-options "-DITERATIONS=1000" { target simulator } }
  #ifndef ITERATIONS
  #define ITERATIONS 50000
  #endif

Jonathan Wakely wrote:
> Adding a testcase so the bug can be closed.
> 
> I believe the segfault was fixed for 3.4.0 by
> https://gcc.gnu.org/r67912
> 
> Tested x86_64-linux, committed to trunk.