[dev.cc] runtime: convert defs_$GOOS_$GOARCH.h to Go

The conversion was done with an automated tool and then
modified only as necessary to make it compile and run.

In a few cases, defs_$GOOS_$GOARCH.go already existed,
so the target here is defs1_$GOOS_$GOARCH.go.

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

LGTM=r
R=r
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/171490043
diff --git a/src/runtime/defs_plan9_386.go b/src/runtime/defs_plan9_386.go
new file mode 100644
index 0000000..170506b
--- /dev/null
+++ b/src/runtime/defs_plan9_386.go
@@ -0,0 +1,23 @@
+package runtime
+
+type ureg struct {
+	di    uint32 /* general registers */
+	si    uint32 /* ... */
+	bp    uint32 /* ... */
+	nsp   uint32
+	bx    uint32 /* ... */
+	dx    uint32 /* ... */
+	cx    uint32 /* ... */
+	ax    uint32 /* ... */
+	gs    uint32 /* data segments */
+	fs    uint32 /* ... */
+	es    uint32 /* ... */
+	ds    uint32 /* ... */
+	trap  uint32 /* trap _type */
+	ecode uint32 /* error code (or zero) */
+	pc    uint32 /* pc */
+	cs    uint32 /* old context */
+	flags uint32 /* old flags */
+	sp    uint32
+	ss    uint32 /* old stack segment */
+}