runtime: convert forcegc helper to Go
Also fix a bunch of bugs:
1. Accesses to last_gc must be atomic (it's int64).
2. last_gc still can be 0 during first checks in sysmon, check for 0.
3. forcegc.g can be unitialized when sysmon accesses it:
        forcegc.g is initialized by main goroutine (forcegc.g = newproc1(...)),
        and main goroutine is unsynchronized with both sysmon and forcegc goroutine.
        Initialize forcegc.g in the forcegc goroutine itself instead.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/136770043