lang/libobjc2: Add -fcommon to CFLAGS

Fix the build of lang/libobjc2 after the llvm 11 import by adding -fcommon
to CFLAGS. There are several patches needed to get this to build with
-fno-common, and I'm unsure about some of the changes I needed to make, so
build with -fcommon for now, until upstream can have a look as well.

MFH: 2020Q3 (implicit, -fno-common fixes, ok by joenum)