Add support for OpenBSD to build with clang, basically link against -…

…lc++abi

since when using clang, the GCC internals aren't around.

I had it in a ugly way hardcoded in the GNUstep ports tree for years.

Tested on amd64, with gcc 11 from packages, and linking against gcc libobjc,
as well as using base clang, and building against libobjc2 from packages.

target.make

@@ -724,8 +724,13 @@ SHARED_LIBEXT   = .so
 
 HAVE_BUNDLES    = yes
 BUNDLE_LD	= $(LD)
+ifeq ($(CLANG_CC), yes)
+BUNDLE_LDFLAGS  += -shared -fPIC -lc++abi
+ADDITIONAL_LDFLAGS += -Wl,-E -lc++abi
+else
 BUNDLE_LDFLAGS  += -shared -fPIC
 ADDITIONAL_LDFLAGS += -Wl,-E
+endif
 STATIC_LDFLAGS += -static
 
 # nm on OpenBSD is rather like on Darwin