cmd/cgo: use standard C syntax for complex types

(instead of using a GCC extension).

Change-Id: I110dc45bfe5f1377fe3453070eccde283b5cc161
Reviewed-on: https://go-review.googlesource.com/17716Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/cgo/out.go

@@ -1420,8 +1420,8 @@ typedef GoUintGOINTBITS GoUint;
 typedef __SIZE_TYPE__ GoUintptr;
 typedef float GoFloat32;
 typedef double GoFloat64;
-typedef __complex float GoComplex64;
-typedef __complex double GoComplex128;
+typedef float _Complex GoComplex64;
+typedef double _Complex GoComplex128;
 
 /*
   static assertion to make sure the file is being used on architecture