runtime: implement cheaper context switch on Linux/AMD64

runtime: implement cheaper context switch on Linux/AMD64

    Currently, goroutine switches are implemented with libc
    getcontext/setcontext functions, which saves/restores the machine
    register states and also the signal context. This does more than
    what we need, and performs an expensive syscall.

    This CL implements a simplified version of getcontext/setcontext,
    in assembly, that only saves/restores the necessary part, i.e.
    the callee-save registers, and the PC, SP. A simplified version
    of makecontext, written in C, is also added. Currently this is
    only implemented on Linux/AMD64.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/178298

From-SVN: r271818