MFH: r545235

devel/libunwind: Fix build with -fno-common

Fix the build of devel/libunwind with -fno-common (the default on FreeBSD
current after the llvm 11 update).
Add an upstream patch (pulling it from the official github mirror) to fix
the build woth -fno-common.

Approved by: ports-secteam (joenum)