STM test failing on Armhf/Linux

During validation on armhf/linux, I found that test T7815 had failed.

Unfortunately, it only fails intermittently on one quad core Arm board and not at all on another quad core Arm board. If I do 10 runs of the test like:

for x in $(seq 1 10) ; do testsuite/tests/rts/T7815 50000 +RTS -N2 ; echo $? ; done

one will fail at least 4 or 5 times and ocassionally as many as 9 or 10 times.

The two boards are:

• Inforce Computing ifc6540 with a Qualcomm Snapdragon 805 CPU.
• Radxa Rock with a Rockchip RK3199 CPU.

The ifc6540 is the one that fails.

@fryguybob suggests that this is actually a bug in the STM implementation that breaks on Arm because of Arm's weaker memory consistency model.