runtime: break out GC pacer into its own file

This change breaks out the GC pacer into its own file so that it'll be
easier to see the full implementation and change it. It also suggests an
obvious place to put tests (mgcpacer_test.go).

This includes all of gcControllerState, gcSetTriggerRatio, anything
related to GOGC, and all related globals and constants.

This is almost a clean move, except that globals and constants are
formatted into blocks instead of having a separate "var" declaration for
each one.

For #44167.

Change-Id: I85aa84ce85c6cfbe0b33e8a3c91cbe9dc41de8cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/306596


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>