runtime: rewrite interface code into Go

runtime: rewrite interface code into Go

    I started to copy the Go 1.7 interface code, but the gc and gccgo
    representations of interfaces are too different.  So instead I rewrote
    the gccgo interface code from C to Go.  The code is largely the same as
    it was, but the names are more like those used in the gc runtime.

    I also copied over the string comparison functions, and tweaked the
    compiler to use eqstring when comparing strings for equality.

    Reviewed-on: https://go-review.googlesource.com/31591

From-SVN: r241384