runtime: clean up GC code
Remove C version of GC.
Convert freeOSMemory to Go.
Restore g0 check in GC.
Remove unknownGCPercent check in GC,
it's initialized explicitly now.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://codereview.appspot.com/139910043