runtime/internal/sys: new package, API copied from Go 1.7

runtime/internal/sys: new package, API copied from Go 1.7

    Copy over the Go 1.7 runtime/internal/sys package, but instead of having
    separate files for each GOARCH and GOOS value, set the values in
    configure.ac and write them out in Makefile.am.  Setting the values in
    configure.ac should make it easier to add new processors.

    Remove the automake GOARCH conditionals, which are no longer used.
    Leave the GOOS conditionals for now, as they are used for the C runtime
    package.

    Reviewed-on: https://go-review.googlesource.com/29018

From-SVN: r240083