getg

Export the runtime.getg() which is unexported by default.

This makes goroutine local storage easier to implement, like in testing environment.

Don't use this in production.

How rewrite of standard lib works?

See project/rewrite_std_test.go for example,basically:

• after loading and AST inspecting, before copying files, set rewriteStd to true
• in GenOverlay phase, gen extra file aside to GOROOT/src/runtime package

This technique does not need to inspect the runtime's AST, so we don't change the ShouldVisitPackage options, instead, we generate the file in the GenOverlay phase.