Advise operating system about memory access during GC.

Advise operating system about memory access during GC.

- Expect sequential access to oldspace during gc_scan_oldspace.
- Free heap pages (newspace) before copying tospace over them.
- Free tospace pages after saving tospace.

The OS hooks are implemented only for Unix, in terms of posix_madvise
and madvise.  Implementing them for Windows and/or OS/2 is left as an
exercise for the reader.