sim: Fix nuttx.rel for macOS

* ld doesn't have --start-groupi/--end-group things.  As far as I know,
  it works that way by default.

* objcopy with Mach-O support is not widely available.
  (GNU binutils seem to claim the support but it didn't actually work
  for me.  llvm-objcopy --redefine-syms explicitly rejects Mach-O.)
  Instead, use -unexported_symbols_list linker flag to hide symbols
  to avoid conflicts with host symbols.

arch/sim/src/Makefile

@@ -195,8 +195,15 @@ endif
 
 # Override in Make.defs if linker is not 'ld'
 
-LDSTARTGROUP ?= --start-group
-LDENDGROUP ?= --end-group
+ifeq ($(HOSTOS),Darwin)
+  LDSTARTGROUP ?=
+  LDENDGROUP ?=
+  LDUNEXPORTSYMBOLS ?= -unexported_symbols_list $(HOSTOS)-names.dat
+else
+  LDSTARTGROUP ?= --start-group
+  LDENDGROUP ?= --end-group
+  LDUNEXPORTSYMBOLS ?=
+endif
 
 EXTRA_LIBS ?=
 EXTRA_LIBPATHS ?=
@@ -292,10 +299,15 @@ else
 	$(Q) cp nuttx-names.dat $@
 endif
 
+Darwin-names.dat: nuttx-names.dat
+	$(Q) cat $^ | sed -e "s/^\([^[:space:]][^[:space:]]*\)[[:space:]].*/_\1/g" >$@
+
 nuttx.rel : libarch$(LIBEXT) board/libboard$(LIBEXT) $(HOSTOS)-names.dat $(LINKOBJS)
 	$(Q) echo "LD:  nuttx.rel"
-	$(Q) $(LD) -r $(LDLINKFLAGS) $(RELPATHS) $(EXTRA_LIBPATHS) -o $@ $(REQUIREDOBJS) $(LDSTARTGROUP) $(RELLIBS) $(EXTRA_LIBS) $(LDENDGROUP)
+	$(Q) $(LD) -r $(LDLINKFLAGS) $(RELPATHS) $(EXTRA_LIBPATHS) -o $@ $(REQUIREDOBJS) $(LDSTARTGROUP) $(RELLIBS) $(EXTRA_LIBS) $(LDENDGROUP) $(LDUNEXPORTSYMBOLS)
+ifneq ($(HOSTOS),Darwin)
 	$(Q) $(OBJCOPY) --redefine-syms=$(HOSTOS)-names.dat $@
+endif
 
 # Generate the final NuttX binary by linking the host-specific objects with the NuttX
 # specific objects (with munged names)
@@ -327,7 +339,7 @@ export_startup: board/libboard$(LIBEXT) up_head.o $(HOSTOBJS)
 depend: .depend
 
 cleanrel:
-	$(Q) rm -f nuttx.rel GNU/Linux-names.dat Cygwin-names.dat
+	$(Q) rm -f nuttx.rel $(HOSTOS)-names.dat
 
 clean: cleanrel
 	$(Q) if [ -e board/Makefile ]; then \