riscv64: add __riscv_flush_icache

Always use the systemcall as uClibc-ng has no vdso support.
Tested with libffi and python in qemu-system-riscv64.

libc/sysdeps/linux/riscv64/Makefile.arch

@@ -1,2 +1,2 @@
-CSRC-y := __syscall_error.c
+CSRC-y := __syscall_error.c cache.c
 SSRC-y := __longjmp.S setjmp.S vfork.S clone.S

libc/sysdeps/linux/riscv64/cache.c

@@ -0,0 +1,51 @@
+/* RISC-V instruction cache flushing VDSO calls
+   Copyright (C) 2017-2020 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <atomic.h>
+#include <sys/syscall.h>
+
+typedef int (*func_type) (void *, void *, unsigned long int);
+
+static int
+__riscv_flush_icache_syscall (void *start, void *end, unsigned long int flags)
+{
+  return INLINE_SYSCALL (riscv_flush_icache, 3, start, end, flags);
+}
+
+static func_type
+__lookup_riscv_flush_icache (void)
+{
+  /* always call the system call directly.*/
+  return &__riscv_flush_icache_syscall;
+}
+
+int
+__riscv_flush_icache (void *start, void *end, unsigned long int flags)
+{
+  static volatile func_type cached_func;
+
+  func_type func = atomic_load_relaxed (&cached_func);
+
+  if (!func)
+    {
+      func = __lookup_riscv_flush_icache ();
+      atomic_store_relaxed (&cached_func, func);
+    }
+
+  return func (start, end, flags);
+}