runtime: merge mallocgc, gomallocgc

I assumed they were the same when I wrote
cgocallback.go earlier today. Merge them
to eliminate confusion.

I can't tell what gomallocgc did before with
a nil type but without FlagNoScan.
I created a call like that in cgocallback.go
this morning, translating from a C file.
It was supposed to do what the C version did,
namely treat the block conservatively.
Now it will.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/141810043

src/runtime/panic.go

@@ -147,7 +147,7 @@ func newdefer(siz int32) *_defer {
 	if d == nil {
 		// deferpool is empty or just a big defer
 		total := goroundupsize(totaldefersize(uintptr(siz)))
-		d = (*_defer)(gomallocgc(total, conservative, 0))
+		d = (*_defer)(mallocgc(total, conservative, 0))
 	}
 	d.siz = siz
 	d.special = false