runtime: move sys.DefaultGoroot to runtime.defaultGOROOT

The default GOROOT has nothing to do with system details.
Move it next to its one use in package runtime.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I1a601fad6335336b4616b834bb21bd8437ee1313
Reviewed-on: https://go-review.googlesource.com/c/go/+/288796


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>