[dev.fuzz] testing: add basic go command support for fuzzing

[dev.fuzz] testing: add basic go command support for fuzzing

This change adds support for a -fuzz flag in the go command, and sets up
the groundwork for native fuzzing support. These functions are no-ops
for now, but will be built out and tested in future PRs.

Change-Id: I58e78eceada5799bcb73acc4ae5a20372badbf40
Reviewed-on: https://go-review.googlesource.com/c/go/+/251441
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>