mono: backport patches to fix PowerPC build

Mono 3.12.0 doesn't build properly on PowerPC, but there are already
patches upstream to fix those issues. This commit backports the two
necessary patches to get the PowerPC build working.

Fixes:

  http://autobuild.buildroot.org/results/91d/91d4b9d3f2d3597b071a1bcec8339eaf2a9c7981/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>