nptl/tst-thread-exit-clobber failure on armv7l-*-linux-gnueabihf

[Florian Weimer]

The test fails because libstdc++ (probably the personality routine)
clobbers the d8 VFP register, which is callee-saved.

This bug goes away when I downgrade libstdc++ to the GCC 8 version, so I
filed a GCC bug:

  <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89093>

I don't plan to debug this further.

Thanks,
Florian