Work around llvm PR 12623, which makes variables declared with .symver

sometimes disappear from the resulting object file, if compiled with
clang.  In particular, this can lead to errors when building world with
clang and -g, similar to:

  /usr/obj/usr/src/tmp/usr/lib/libc.so: undefined reference to `_malloc_options'

Reported by:	Conrad J. Sabatier <conrads@.no.cox.net>
Reviewed by:	jasone

contrib/jemalloc/src/jemalloc.c

@@ -8,7 +8,8 @@ malloc_tsd_data(, arenas, arena_t *, NULL)
 malloc_tsd_data(, thread_allocated, thread_allocated_t,
     THREAD_ALLOCATED_INITIALIZER)
 
-const char	*__malloc_options_1_0;
+/* Work around <http://llvm.org/bugs/show_bug.cgi?id=12623>: */
+const char	*__malloc_options_1_0 = NULL;
 __sym_compat(_malloc_options, __malloc_options_1_0, FBSD_1.0);
 
 /* Runtime configuration options. */