sync/atomic: FreeBSD/ARM support

only supports ARMv6K and newer ARM cores.

R=rsc, dave
CC=golang-dev
https://golang.org/cl/6601064

src/pkg/sync/atomic/asm_freebsd_arm.s

+// Copyright 2012 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.
+
+// FreeBSD/ARM atomic operations.
+// TODO(minux): this only supports ARMv6K or higher.
+
+TEXT ·CompareAndSwapInt32(SB),7,$0
+	B ·CompareAndSwapUint32(SB)
+
+TEXT ·CompareAndSwapUint32(SB),7,$0
+	B ·armCompareAndSwapUint32(SB)
+
+TEXT ·CompareAndSwapUintptr(SB),7,$0
+	B ·CompareAndSwapUint32(SB)
+
+TEXT ·CompareAndSwapPointer(SB),7,$0
+	B ·CompareAndSwapUint32(SB)
+
+TEXT ·AddInt32(SB),7,$0
+	B ·AddUint32(SB)
+
+TEXT ·AddUint32(SB),7,$0
+	B ·armAddUint32(SB)
+
+TEXT ·AddUintptr(SB),7,$0
+	B ·AddUint32(SB)
+
+TEXT ·CompareAndSwapInt64(SB),7,$0
+	B ·CompareAndSwapUint64(SB)
+
+TEXT ·CompareAndSwapUint64(SB),7,$-4
+	B ·armCompareAndSwapUint64(SB)
+
+TEXT ·AddInt64(SB),7,$0
+	B ·addUint64(SB)
+
+TEXT ·AddUint64(SB),7,$0
+	B ·addUint64(SB)
+
+TEXT ·LoadInt32(SB),7,$0
+	B ·LoadUint32(SB)
+
+TEXT ·LoadUint32(SB),7,$0
+	MOVW addr+0(FP), R1
+load32loop:
+	LDREX (R1), R2		// loads R2
+	STREX R2, (R1), R0	// stores R2
+	CMP $0, R0
+	BNE load32loop
+	MOVW R2, val+4(FP)
+	RET
+
+TEXT ·LoadInt64(SB),7,$0
+	B ·loadUint64(SB)
+
+TEXT ·LoadUint64(SB),7,$0
+	B ·loadUint64(SB)
+
+TEXT ·LoadUintptr(SB),7,$0
+	B ·LoadUint32(SB)
+
+TEXT ·LoadPointer(SB),7,$0
+	B ·LoadUint32(SB)
+
+TEXT ·StoreInt32(SB),7,$0
+	B ·StoreUint32(SB)
+
+TEXT ·StoreUint32(SB),7,$0
+	MOVW addr+0(FP), R1
+	MOVW val+4(FP), R2
+storeloop:
+	LDREX (R1), R4		// loads R4
+	STREX R2, (R1), R0	// stores R2
+	CMP $0, R0
+	BNE storeloop
+	RET
+
+TEXT ·StoreInt64(SB),7,$0
+	B ·storeUint64(SB)
+
+TEXT ·StoreUint64(SB),7,$0
+	B ·storeUint64(SB)
+
+TEXT ·StoreUintptr(SB),7,$0
+	B ·StoreUint32(SB)
+
+TEXT ·StorePointer(SB),7,$0
+	B ·StoreUint32(SB)