Increased malloc margin to 128.

arduino/Arduino#1329 (comment)
See #857 #1329

Author: cmaglie

cores/arduino/avr-libc/malloc.c

@@ -52,7 +52,7 @@
 
 /* May be changed by the user only before the first malloc() call.  */
 
-size_t __malloc_margin = 32;
+size_t __malloc_margin = 128;
 char *__malloc_heap_start = &__heap_start;
 char *__malloc_heap_end = &__heap_end;