Currently the runtime allocates workbufs from persistent memory, which
means they can never be freed.

Switch to allocating them from manually-managed heap spans. This
doesn't free them yet, but it puts us in a position to do so.

For #19325.

Change-Id: I94b2512a2f2bbbb456cd9347761b9412e80d2da9
Reviewed-on: https://go-review.googlesource.com/38581
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>