[compiler-rt] Implement __clear_cache() on OpenBSD/riscv64

(cherry picked from commit 3db9590)

Author: jcourreges

compiler-rt/lib/builtins/clear_cache.c

@@ -35,7 +35,7 @@ uintptr_t GetCurrentProcess(void);
 #include <machine/sysarch.h>
 #endif
 
-#if defined(__OpenBSD__) && (defined(__arm__) || defined(__mips__))
+#if defined(__OpenBSD__) && (defined(__arm__) || defined(__mips__) || defined(__riscv))
 // clang-format off
 #include <sys/types.h>
 #include <machine/sysarch.h>
@@ -166,6 +166,13 @@ void __clear_cache(void *start, void *end) {
                    : "=r"(start_reg)
                    : "r"(start_reg), "r"(end_reg), "r"(flags), "r"(syscall_nr));
   assert(start_reg == 0 && "Cache flush syscall failed.");
+#elif defined(__riscv) && defined(__OpenBSD__)
+  struct riscv_sync_icache_args arg;
+
+  arg.addr = (uintptr_t)start;
+  arg.len = (uintptr_t)end - (uintptr_t)start;
+
+  sysarch(RISCV_SYNC_ICACHE, &arg);
 #else
 #if __APPLE__
   // On Darwin, sys_icache_invalidate() provides this functionality