Fix fread failure after enable_incremental if malloc redirected (Cher…

…iBSD)

Issue #627 (bdwgc).

The failure is observed in cord `refill_cache()` executed by
`cordtest`, `fread()` fails with `EFAULT`.  The fix is to disable
` on FreeBSD when `REDIRECT_MALLOC` is defined (similar
as it was done previously on Linux).

* include/private/gcconfig.h [FREEBSD && !ALPHA && !SPARC]
(MPROTECT_VDB): Do not define if `REDIRECT_MALLOC`.

include/private/gcconfig.h

@@ -891,7 +891,11 @@ extern int _modules_data_start[], _apps_bss_end[];
 extern char etext[];
 #    define DATASTART GC_SysVGetDataStart(0x1000, (ptr_t)etext)
 #    define DATASTART_USES_XGETDATASTART
-#    define MPROTECT_VDB
+#    ifndef REDIRECT_MALLOC
+#      define MPROTECT_VDB
+#    else
+/* Similar as on Linux, fread() might use malloc(). */
+#    endif
 #  endif
 #endif /* FREEBSD */