go.sys: copy files from syscall package to go.sys/{plan9,windows,unix}

This CL copies to each package of go.sys the files from syscall it will need.
Different directories have different files, but these:
        mkall.sh
        str.go
        syscall.go
        mksyscall.pl
        race.go
        race0.go
        syscall_test.go
are copied to all three.
No changes yet, these are just copies. They are not ready to use yet:
package names are wrong, for starters. But this clean copy will make
it easier to follow the changes as the packages are enabled.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126960043

unix/asm_nacl_386.s

@@ -0,0 +1,43 @@
+// Copyright 2013 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 "../../cmd/ld/textflag.h"
+#include "../runtime/syscall_nacl.h"
+
+//
+// System call support for 386, Native Client
+//
+
+#define NACL_SYSCALL(code) \
+	MOVL $(0x10000 + ((code)<<5)), AX; CALL AX
+
+#define NACL_SYSJMP(code) \
+	MOVL $(0x10000 + ((code)<<5)), AX; JMP AX
+
+TEXT syscall·Syscall(SB),NOSPLIT,$12-28
+	CALL	runtime·entersyscall(SB)
+	MOVL	trap+0(FP), AX
+	MOVL	a1+4(FP), BX
+	MOVL	BX, 0(SP)
+	MOVL	a2+8(FP), BX
+	MOVL	BX, 4(SP)
+	MOVL	a3+12(FP), BX
+	MOVL	BX, 8(SP)
+	SHLL	$5, AX
+	ADDL	$0x10000, AX
+	CALL	AX
+	CMPL	AX, $0
+	JGE	ok
+	MOVL	$-1, r1+16(FP)
+	MOVL	$-1, r2+20(FP)
+	NEGL	AX
+	MOVL	AX, err+24(FP)
+	CALL	runtime·exitsyscall(SB)
+	RET
+ok:
+	MOVL	AX, r1+16(FP)
+	MOVL	DX, r2+20(FP)
+	MOVL	$0, err+24(FP)
+	CALL	runtime·exitsyscall(SB)
+	RET