runtime: support MADV_DONTNEED on the BSDs and Solaris/Illumos

runtime: support MADV_DONTNEED on the BSDs and Solaris/Illumos

Like on Linux, use GODEBUG=madvdontneed=1 to enable madvise with
MADV_DONTNEED instead of MADV_FREE.

Based on previous work by Marcelo Araujo in CL 181498.

Fixes #32519

Change-Id: Ib49faf05c42a65591b90e75f663146f213030529
Reviewed-on: https://go-review.googlesource.com/c/go/+/266937
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>