bug#21620: GMP 6.0.0a intermittent test failure on armhf

[Ludovic Courtès]

‘tests/mpz/reuse’ intermittently fails on armhf:

  http://hydra.gnu.org/build/704686/nixlog/1/raw

Running this program in a loop easily reproduces the problem, but not
deterministically (different parts fail.)

Ludo’.