package/openmpi: fix build failure due to Gcc Bug 68485

Package openmpi manifests Microblaze Gcc Bug 68485 resulting in a build
failure due to an Internal Compiler Error.

As done for other packages in Buildroot work around this Gcc Bug by
setting optimization to -O0 if BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y.

Fixes:
http://autobuild.buildroot.net/results/8f3/8f334427e7475154d69469f8ee4efab6df80e403/



Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>