riscv64 <sys/user.h>.

Empty, at least for now. The other libcs seem not to have invented types
here, so let's see whether there's really a call for them before we do.

Test: treehugger
Change-Id: Icbace2c02abc011388edc8e3e238bb64c640feb7

libc/include/sys/user.h

@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _SYS_USER_H_
-#define _SYS_USER_H_
+#pragma once
 
 #include <sys/cdefs.h>
 #include <stddef.h> /* For size_t. */
@@ -233,6 +232,11 @@ struct user_fpsimd_struct {
   uint32_t fpcr;
 };
 
+#elif defined(__riscv)
+
+// This space deliberately left blank for now.
+// No other libcs have any riscv64-specific structs.
+
 #else
 
 #error "Unsupported architecture."
@@ -240,5 +244,3 @@ struct user_fpsimd_struct {
 #endif
 
 __END_DECLS
-
-#endif  /* _SYS_USER_H_ */