runtime: add GC CPU utilization limiter

This change adds a GC CPU utilization limiter to the GC. It disables
assists to ensure GC CPU utilization remains under 50%. It uses a leaky
bucket mechanism that will only fill if GC CPU utilization exceeds 50%.
Once the bucket begins to overflow, GC assists are limited until the
bucket empties, at the risk of GC overshoot. The limiter is primarily
updated by assists. The scheduler may also update it, but only if the
GC is on and a few milliseconds have passed since the last update. This
second case exists to ensure that if the limiter is on, and no assists
are happening, we're still updating the limiter regularly.

The purpose of this limiter is to mitigate GC death spirals, opting to
use more memory instead.

This change turns the limiter on always. In practice, 50% overall GC CPU
utilization is very difficult to hit unless you're trying; even the most
allocation-heavy applications with complex heaps still need to do
something with that memory. Note that small GOGC values (i.e.
single-digit, or low teens) are more likely to trigger the limiter,
which means the GOGC tradeoff may no longer be respected. Even so, it
should still be relatively rare.

This change also introduces the feature flag for code to support the
memory limit feature.

For #48409.

Change-Id: Ia30f914e683e491a00900fd27868446c65e5d3c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/353989
Reviewed-by: Michael Pratt <mpratt@google.com>

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