malloc: Use compat_symbol_reference in libmcheck [BZ #22050]

Since glibc 2.24, __malloc_initialize_hook is a compat symbol.  As a
result, the link editor does not export a definition of
__malloc_initialize_hook from the main program, so that it no longer
interposes the variable definition in libc.so.  Specifying the symbol
version restores the exported symbol.

malloc/mcheck-init.c

@@ -20,6 +20,7 @@
 
 #include <malloc.h>
 #include <mcheck.h>
+#include <shlib-compat.h>
 
 static void
 turn_on_mcheck (void)
@@ -28,3 +29,5 @@ turn_on_mcheck (void)
 }
 
 void (*__malloc_initialize_hook) (void) = turn_on_mcheck;
+compat_symbol_reference (libc, __malloc_initialize_hook,
+                         __malloc_initialize_hook, GLIBC_2_0);