riscv: add asm/unistd.h UAPI header

Marcin Juszkiewicz reported issues while generating syscall table for riscv
using 4.20-rc1. The patch refactors our unistd.h files to match some other
architectures.

- Add asm/unistd.h UAPI header, which has __ARCH_WANT_NEW_STAT only for 64-bit
- Remove asm/syscalls.h UAPI header and merge to asm/unistd.h
- Adjust kernel asm/unistd.h

So now asm/unistd.h UAPI header should show all syscalls for riscv.

Before this, Makefile simply put `#include <asm-generic/unistd.h>` into
generated asm/unistd.h UAPI header thus user didn't see:

- __NR_riscv_flush_icache
- __NR_newfstatat
- __NR_fstat

which are supported by riscv kernel.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Fixes: 67314ec7b0 ("RISC-V: Request newstat syscalls")
Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/uapi/asm/unistd.h

@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Copyright (C) 2018 David Abdurachmanov <david.abdurachmanov@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifdef __LP64__
+#define __ARCH_WANT_NEW_STAT
+#endif /* __LP64__ */
+
+#include <asm-generic/unistd.h>
+
+/*
+ * Allows the instruction cache to be flushed from userspace.  Despite RISC-V
+ * having a direct 'fence.i' instruction available to userspace (which we
+ * can't trap!), that's not actually viable when running on Linux because the
+ * kernel might schedule a process on another hart.  There is no way for
+ * userspace to handle this without invoking the kernel (as it doesn't know the
+ * thread->hart mappings), so we've defined a RISC-V specific system call to
+ * flush the instruction cache.
+ *
+ * __NR_riscv_flush_icache is defined to flush the instruction cache over an
+ * address range, with the flush applying to either all threads or just the
+ * caller.  We don't currently do anything with the address range, that's just
+ * in there for forwards compatibility.
+ */
+#ifndef __NR_riscv_flush_icache
+#define __NR_riscv_flush_icache (__NR_arch_specific_syscall + 15)
+#endif
+__SYSCALL(__NR_riscv_flush_icache, sys_riscv_flush_icache)