[dev.cc] runtime: convert Plan 9 port to Go

Thanks to Aram Hăvărneanu, Nick Owens
and Russ Cox for the early reviews.

LGTM=aram, rsc
R=rsc, lucio.dere, aram, ality
CC=golang-codereviews, mischief
https://golang.org/cl/175370043

src/runtime/os2_plan9.go

@@ -0,0 +1,72 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Plan 9-specific system calls
+
+package runtime
+
+// open
+const (
+	_OREAD   = 0
+	_OWRITE  = 1
+	_ORDWR   = 2
+	_OEXEC   = 3
+	_OTRUNC  = 16
+	_OCEXEC  = 32
+	_ORCLOSE = 64
+	_OEXCL   = 0x1000
+)
+
+// rfork
+const (
+	_RFNAMEG  = 1 << 0
+	_RFENVG   = 1 << 1
+	_RFFDG    = 1 << 2
+	_RFNOTEG  = 1 << 3
+	_RFPROC   = 1 << 4
+	_RFMEM    = 1 << 5
+	_RFNOWAIT = 1 << 6
+	_RFCNAMEG = 1 << 10
+	_RFCENVG  = 1 << 11
+	_RFCFDG   = 1 << 12
+	_RFREND   = 1 << 13
+	_RFNOMNT  = 1 << 14
+)
+
+// notify
+const (
+	_NCONT = 0
+	_NDFLT = 1
+)
+
+type uinptr _Plink
+
+type tos struct {
+	prof struct { // Per process profiling
+		pp    *_Plink // known to be 0(ptr)
+		next  *_Plink // known to be 4(ptr)
+		last  *_Plink
+		first *_Plink
+		pid   uint32
+		what  uint32
+	}
+	cyclefreq uint64 // cycle clock frequency if there is one, 0 otherwise
+	kcycles   int64  // cycles spent in kernel
+	pcycles   int64  // cycles spent in process (kernel + user)
+	pid       uint32 // might as well put the pid here
+	clock     uint32
+	// top of stack is here
+}
+
+const (
+	_NSIG   = 14  // number of signals in sigtable array
+	_ERRMAX = 128 // max length of note string
+
+	// Notes in runtime·sigtab that are handled by runtime·sigpanic.
+	_SIGRFAULT = 2
+	_SIGWFAULT = 3
+	_SIGINTDIV = 4
+	_SIGFLOAT  = 5
+	_SIGTRAP   = 6
+)