runtime: default to MADV_DONTNEED on Linux

In Go 1.12, we changed the runtime to use MADV_FREE when available on
Linux (falling back to MADV_DONTNEED) in CL 135395 to address issue
 #23687. While MADV_FREE is somewhat faster than MADV_DONTNEED, it
doesn't affect many of the statistics that MADV_DONTNEED does until
the memory is actually reclaimed under OS memory pressure. This
generally leads to poor user experience, like confusing stats in top
and other monitoring tools; and bad integration with management
systems that respond to memory usage.

We've seen numerous issues about this user experience, including
 #41818, #39295, #37585, #33376, and #30904, many questions on Go
mailing lists, and requests for mechanisms to change this behavior at
run-time, such as #40870. There are also issues that may be a result
of this, but root-causing it can be difficult, such as #41444 and
 #39174. And there's some evidence it may even be incompatible with
Android's process management in #37569.

This CL changes the default to prefer MADV_DONTNEED over MADV_FREE, to
favor user-friendliness and minimal surprise over performance. I think
it's become clear that Linux's implementation of MADV_FREE ultimately
doesn't meet our needs. We've also made many improvements to the
scavenger since Go 1.12. In particular, it is now far more prompt and
it is self-paced, so it will simply trickle memory back to the system
a little more slowly with this change. This can still be overridden by
setting GODEBUG=madvdontneed=0.

Fixes #42330 (meta-issue).

Fixes #41818, #39295, #37585, #33376, #30904 (many of which were
already closed as "working as intended").

Change-Id: Ib6aa7f2dc8419b32516cc5a5fc402faf576c92e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/267100


Trust: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>