I found a toolchain with 5.3.0, and using it the problem disappears. Oh well.It would be useful to know what compiler version was used to build the4.6.3 from kernel.org.
kernel. I wouldn't be surprised if some are buggy.
That is utterly ancient and probaby very buggy. I would recommend 5.x+
or at the very least 4.7 or 4.8.