A workaround for macOS linker

It seems that "ld -r" on macOS doesn't include objects from
libraries for common symbols. Because of that, sim build
ends up with undefined references to globals like g_binfmts
and g_mmheap.

	@(#)PROGRAM:ld  PROJECT:ld64-530
	BUILD 18:57:17 Dec 13 2019
	configured to support archs: armv6 armv7 armv7s arm64 arm64e arm64_32 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em
	LTO support using: LLVM version 11.0.0, (clang-1100.0.33.17) (static support for 23, runtime is 23)
	TAPI support using: Apple TAPI version 11.0.0 (tapi-1100.0.11)

binfmt/binfmt_globals.c

@@ -56,7 +56,7 @@
  * protection to simply disable pre-emption when accessing this list.
  */
 
-FAR struct binfmt_s *g_binfmts;
+FAR struct binfmt_s *g_binfmts = NULL;
 
 /****************************************************************************
  * Private Functions

mm/umm_heap/umm_globals.c

@@ -64,5 +64,5 @@
 #else
 /* Otherwise, the user heap data structures are in common .bss */
 
-struct mm_heap_s g_mmheap;
+struct mm_heap_s g_mmheap = {};
 #endif