reflect: add tests for variadic method calls

These tests fail when using gccgo.  In gccgo using Interface
on the value of a method function is implemented using a
variant of MakeFunc.  That approach did not correctly handle
variadic functions.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/151280043
runtime: delay freeing of shrunk stacks until gc is done.

This change prevents confusion in the garbage collector.
The collector wants to make sure that every pointer it finds
isn't junk.  Its criteria for junk is (among others) points
to a "free" span.

Because the stack shrinker modifies pointers in the heap,
there is a race condition between the GC scanner and the
shrinker.  The GC scanner can see old pointers (pointers to
freed stacks).  In particular this happens with SudoG.elem
pointers.

Normally this is not a problem, as pointers into stack spans
are ok.  But if the freed stack is the last one in its span,
the span is marked as "free" instead of "contains stacks".

This change makes sure that even if the GC scanner sees
an old pointer, the span into which it points is still
marked as "contains stacks", and thus the GC doesn't
complain about it.

This change will make the GC pause a tiny bit slower, as
the stack freeing now happens in serial with the mark pause.
We could delay the freeing until the mutators start back up,
but this is the simplest change for now.

TBR=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/158750043
reflect: add direct call tests to TestMakeFuncVariadic

TestMakeFuncVariadic only called the variadic function via Call and
CallSlice, not via a direct function call.

I thought these tests would fail under gccgo tip, but they don't.  Still seems worth having though.

LGTM=iant
R=golang-codereviews, gobot, iant
CC=golang-codereviews
https://codereview.appspot.com/152060043

Committer: Ian Lance Taylor <iant@golang.org>