cmd/cgo: use _Complex in rewritten Go code too

Followup to CL 17716, which updated cgo's boilerplate prologue code to
use standard C's _Complex instead of GCC's __complex extension.

Change-Id: I74f29b0cc3d13cab2853441cafbfe77853bba4f9
Reviewed-on: https://go-review.googlesource.com/17820
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/gcc.go

@@ -38,8 +38,8 @@ var nameToC = map[string]string{
 	"ulong":         "unsigned long",
 	"longlong":      "long long",
 	"ulonglong":     "unsigned long long",
-	"complexfloat":  "__complex float",
-	"complexdouble": "__complex double",
+	"complexfloat":  "float _Complex",
+	"complexdouble": "double _Complex",
 }
 
 // cname returns the C name to use for C.s.