decrease fiber stack size. 1MB is too large for windows. [Bug #6344]

[ruby-dev:45554]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35697 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -49,7 +49,7 @@
 static long pagesize;
 
  #if SIZEOF_VOIDP==8
-  #define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x100000)
+  #define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x20000)
  #else
   #define FIBER_MACHINE_STACK_ALLOCATION_SIZE  (0x10000)
  #endif