runtime: expose the g variable

runtime: expose the g variable

    Currently, getg is implemented in C, which loads the thread-local
    g variable. The g variable is declared static in C.

    This CL exposes the g variable, so it can be accessed from the Go
    side. This allows the Go compiler to inline getg calls to direct
    access of g.

    Currently, the actual inlining is only implemented in the gollvm
    compiler. The g variable is thread-local and the compiler backend
    may choose to cache the TLS address in a register or on stack. If
    a thread switch happens the cache may become invalid. I don't
    know how to disable the TLS address cache in gccgo, therefore
    the inlining of getg is not implemented. In the future gccgo may
    gain this if we know how to do it safely.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/186238

From-SVN: r273499