Memory management was consolitated with the BSD ports, since
it was almost identical.

Assembly thunks are gone, being replaced by the new //go:linkname
feature.

This change supersedes CL 138390043 (runtime: convert solaris
netpoll to Go), which was previously reviewed and tested.

This change is only the first step, the port now builds,
but doesn't run. Binaries fail to exec:

    ld.so.1: 6.out: fatal: 6.out: TLS requirement failure : TLS support is unavailable
    Killed

This seems to happen because binaries don't link with libc.so
anymore. We will have to solve that in a different CL.

Also this change is just a rough translation of the original
C code, cleanup will come in a different CL.

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

LGTM=rsc
R=rsc, dave
CC=golang-codereviews, iant, khr, minux, r, rlh
https://golang.org/cl/174960043