small embedded target for arm.

R=rsc
CC=golang-dev
https://golang.org/cl/193104

src/pkg/runtime/embedded/README

+small embedded target for arm
+define the c function write to make debug output work
+
+

src/pkg/runtime/embedded/arm/defs.h

+// nothing to see here

src/pkg/runtime/embedded/arm/rt0.s

+// nothing to see here

src/pkg/runtime/embedded/arm/signal.c

+// nothing to see here

src/pkg/runtime/embedded/arm/sys.s

+// nothing to see here

src/pkg/runtime/embedded/mem.c

+// 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.
+
+#include "runtime.h"
+#include "malloc.h"
+
+// Assume there's an arbitrary amount of memory starting at "end".
+
+void*
+SysAlloc(uintptr ask)
+{
+	static byte *p;
+	extern byte end[];
+	byte *q;
+	
+	if(p == nil) {
+		p = end;
+		p += 7 & -(uintptr)p;
+	}
+	ask += 7 & -ask;
+
+	q = p;
+	p += ask;
+	·memclr(q, ask);
+	return q;
+}
+
+void
+SysFree(void *v, uintptr n)
+{
+	USED(v, n);
+}
+
+void
+SysUnused(void *v, uintptr n)
+{
+	USED(v, n);
+}
+

src/pkg/runtime/embedded/os.h

+// nothing to see here

src/pkg/runtime/embedded/thread.c

+// 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.
+
+#include "runtime.h"
+
+int8 *goos = "embedded";
+
+void
+minit(void)
+{
+}
+
+void
+osinit(void)
+{
+}
+
+void
+initsig(void)
+{
+}
+
+void
+exit(int32)
+{
+	for(;;);
+}
+
+// single processor, no interrupts,
+// so no need for real concurrency or atomicity
+
+void
+newosproc(M *m, G *g, void *stk, void (*fn)(void))
+{
+	USED(m, g, stk, fn);
+	throw("newosproc");
+}
+
+void
+lock(Lock *l)
+{
+	if(m->locks < 0)
+		throw("lock count");
+	m->locks++;
+	if(l->key != 0)
+		throw("deadlock");
+	l->key = 1;
+}
+
+void
+unlock(Lock *l)
+{
+	m->locks--;
+	if(m->locks < 0)
+		throw("lock count");
+	if(l->key != 1)
+		throw("unlock of unlocked lock");
+	l->key = 0;
+}
+
+void
+noteclear(Note *n)
+{
+	n->lock.key = 0;
+}
+
+void
+notewakeup(Note *n)
+{
+	n->lock.key = 1;
+}
+
+void
+notesleep(Note *n)
+{
+	if(n->lock.key != 1)
+		throw("notesleep");
+}
+