unix: provide linux/riscv64 assembly for syscalls

While the linux/riscv64 port is not yet upstream, most of the syscalls package
already has linux/riscv64 support and this will make testing and development
easier.

Updates golang/go#27532

Change-Id: I6e505be55fdbb2942fb6e4f34c040e7b82809630
Reviewed-on: https://go-review.googlesource.com/c/sys/+/177799
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/asm_linux_riscv64.s

@@ -0,0 +1,54 @@
+// Copyright 2019 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.
+
+// +build riscv64,!gccgo
+
+#include "textflag.h"
+
+//
+// System calls for linux/riscv64.
+//
+// Where available, just jump to package syscall's implementation of
+// these functions.
+
+TEXT ·Syscall(SB),NOSPLIT,$0-56
+	JMP	syscall·Syscall(SB)
+
+TEXT ·Syscall6(SB),NOSPLIT,$0-80
+	JMP	syscall·Syscall6(SB)
+
+TEXT ·SyscallNoError(SB),NOSPLIT,$0-48
+	CALL	runtime·entersyscall(SB)
+	MOV	a1+8(FP), A0
+	MOV	a2+16(FP), A1
+	MOV	a3+24(FP), A2
+	MOV	$0, A3
+	MOV	$0, A4
+	MOV	$0, A5
+	MOV	$0, A6
+	MOV	trap+0(FP), A7	// syscall entry
+	ECALL
+	MOV	A0, r1+32(FP)	// r1
+	MOV	A1, r2+40(FP)	// r2
+	CALL	runtime·exitsyscall(SB)
+	RET
+
+TEXT ·RawSyscall(SB),NOSPLIT,$0-56
+	JMP	syscall·RawSyscall(SB)
+
+TEXT ·RawSyscall6(SB),NOSPLIT,$0-80
+	JMP	syscall·RawSyscall6(SB)
+
+TEXT ·RawSyscallNoError(SB),NOSPLIT,$0-48
+	MOV	a1+8(FP), A0
+	MOV	a2+16(FP), A1
+	MOV	a3+24(FP), A2
+	MOV	ZERO, A3
+	MOV	ZERO, A4
+	MOV	ZERO, A5
+	MOV	trap+0(FP), A7	// syscall entry
+	ECALL
+	MOV	A0, r1+32(FP)
+	MOV	A1, r2+40(FP)
+	RET