runtime: use #ifdef instead of #if for USING_SPLIT_STACK

    
    USING_SPLIT_STACK is configured as defined/undefined, not 0/1.
    Most of the places test USING_SPLIT_STACK with #ifdef, with a
    few exceptions. This CL fixes the exceptions.
    
    Reviewed-on: https://go-review.googlesource.com/120596

From-SVN: r261980

gcc/go/gofrontend/MERGE

-bdead75ea02fa852a559f35b41453df0c47c9a66
+7008302f1f0eaa9508b2857185505d4dc7baac1e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/proc.c

@@ -716,7 +716,7 @@ runtime_malg(bool allocatestack, bool signalstack, byte** ret_stack, uintptr* re
 	G *newg;
 	byte* unused_stack;
 	uintptr unused_stacksize;
-#if USING_SPLIT_STACK
+#ifdef USING_SPLIT_STACK
 	int dont_block_signals = 0;
 	size_t ss_stacksize;
 #endif
@@ -738,7 +738,7 @@ runtime_malg(bool allocatestack, bool signalstack, byte** ret_stack, uintptr* re
 #endif
 		}
 
-#if USING_SPLIT_STACK
+#ifdef USING_SPLIT_STACK
 		*ret_stack = __splitstack_makecontext(stacksize,
 						      (void*)(&newg->stackcontext[0]),
 						      &ss_stacksize);
@@ -777,7 +777,7 @@ void stackfree(G*)
 void
 stackfree(G* gp)
 {
-#if USING_SPLIT_STACK
+#ifdef USING_SPLIT_STACK
   __splitstack_releasecontext((void*)(&gp->stackcontext[0]));
 #else
   // If gcstacksize is 0, the stack is allocated by libc and will be