Currently when the garbage collector frees stacks of dead goroutines
in markrootFreeGStacks, it calls stackfree on a regular user stack.
This is a problem, since stackfree manipulates the stack cache in the
per-P mcache, so if it grows the stack or gets preempted in the middle
of manipulating the stack cache (which are both possible since it's on
a user stack), it can easily corrupt the stack cache.

Fix this by calling markrootFreeGStacks on the system stack, so that
all calls to stackfree happen on the system stack. To prevent this bug
in the future, mark stack functions that manipulate the mcache as
go:systemstack.

Fixes #15853.

Change-Id: Ic0d1c181efb342f134285a152560c3a074f14a3d
Reviewed-on: https://go-review.googlesource.com/23511
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>