runtime: add GOEXPERIMENT=runtimefree

This CL is part of a series of CLs to triangulate between the runtime,
compiler, and standard library to reduce how much work the GC must do.

An overall design document is in CL 700255.

This CL stack implements a runtime.free within the runtime, and
then uses it via automatic calls inserted by the compiler when
the compiler proves it is safe to do so. In the future, we can
also consider possibly a limited set of explicit calls from certain
low-level portions of the standard library.

When called, runtime.free allows immediate reuse of memory
without waiting for a GC cycle. The goals include less overall
CPU usage by the GC, longer times between GC cycles
(with less overall time with the write barrier enabled),
and more cache-friendly allocations for user code.

Here, we just add the GOEXPERIMENT=runtimefree flag. It currently
defaults to on, but can be disabled with GOEXPERIMENT=noruntimefree.

The actual implementation starts in CL 673695.

Updates #74299

Change-Id: I2f1f04dbdca51f4aaa735fd65bb2719c298d922e
Reviewed-on: https://go-review.googlesource.com/c/go/+/700235
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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