native: fix C++ for FreeBSD

Cross-compiling C++ for 32-bit on a 64-bit FreeBSD is not as easy as
installing the respective lib via package management as it is in most
Linux distributions. This configures the build system to honor the
`libstdcpp` feature for 64-bit FreeBSD systems.

boards/native/Makefile.include

@@ -52,6 +52,9 @@ ifeq ($(OS_ARCH),x86_64)
 endif
 ifeq ($(OS),FreeBSD)
   ifeq ($(OS_ARCH),amd64)
+    ifeq ($(DISABLE_LIBSTDCPP),1)
+      LINKFLAGS += -nodefaultlibs -lgcc_s -lc -lc++ -lm -lpthread
+    endif
     LINKFLAGS += -m32 -DCOMPAT_32BIT -L/usr/lib32 -B/usr/lib32
   endif
   LINKFLAGS += -L $(BINDIR)

cpu/native/Makefile.dep

@@ -1,3 +1,15 @@
+ifeq (FreeBSD,$(OS))
+  ifeq ($(OS_ARCH),amd64)
+    DISABLE_LIBSTDCPP ?= 1
+  endif
+  ifeq ($(DISABLE_LIBSTDCPP),1)
+    # static C++ constructors need guards for thread safe initialization
+    ifneq (,$(filter cpp,$(FEATURES_USED)))
+      USEMODULE += cxx_ctor_guards
+    endif
+  endif
+endif
+
 ifneq (,$(filter periph_spi,$(USEMODULE)))
   USEMODULE += periph_spidev_linux
 endif

cpu/native/Makefile.features

@@ -1,7 +1,3 @@
-ifeq (FreeBSD,$(OS))
-  DISABLE_LIBSTDCPP ?= 1
-endif
-
 FEATURES_PROVIDED += arch_32bit
 FEATURES_PROVIDED += arch_native
 FEATURES_PROVIDED += cpp