update riscv64 syscall numbers to linux v5.1

commit f3f96f2daa4d00f0e38489fb465cd0244b531abe added these for the
rest of the archs, but the patch it corresponded to missed riscv64
since riscv64 was not yet upstream at the time. this caused commit
dfc81828f7ab41da08f744c44117a1bb20a05749 to break riscv64 build, due
to a wrong assumption that SYS_statx was unconditionally defined.

arch/riscv64/bits/syscall.h.in

@@ -273,5 +273,13 @@
 #define __NR_pkey_mprotect 288
 #define __NR_pkey_alloc 289
 #define __NR_pkey_free 290
+#define __NR_statx 291
+#define __NR_io_pgetevents 292
+#define __NR_rseq 293
+#define __NR_kexec_file_load 294
+#define __NR_pidfd_send_signal 424
+#define __NR_io_uring_setup 425
+#define __NR_io_uring_enter 426
+#define __NR_io_uring_register 427
 #define __NR_sysriscv __NR_arch_specific_syscall
 #define __NR_riscv_flush_icache (__NR_sysriscv + 15)