5g, 6g, 8g: flush modified globals aggressively

5g, 6g, 8g: flush modified globals aggressively

The alternative is to record enough information that the
trap handler know which registers contain cached globals
and can flush the registers back to their original locations.
That's significantly more work.

This only affects globals that have been written to.
Code that reads from a global should continue to registerize
as well as before.

Fixes #1304.

R=ken2
CC=golang-dev
https://golang.org/cl/5687046