runtime: copy signal code from Go 1.7 runtime

runtime: copy signal code from Go 1.7 runtime

    Add a little shell script to auto-generate runtime.sigtable from the
    known signal names.

    Force the main package to always import the runtime package.  Otherwise
    some runtime package global variables may never be initialized.

    Set the syscallsp and syscallpc fields of g when entering a syscall, so
    that the runtime package knows when a g is executing a syscall.

    Fix runtime.funcPC to avoid dead store elimination of the interface
    value when the function is inlined.

    Reviewed-on: https://go-review.googlesource.com/33025

From-SVN: r242060