unix: add riscv_hwprobe for riscv64

The riscv_hwprobe system call was introduced in Linux 6.4 and allows
the caller to determine a number of interesting pieces of information
about the underlying RISC-V CPUs, e.g., which extensions they support
and whether they allow fast unaligned memory accesses.  For more information
please see:

https://docs.kernel.org/riscv/hwprobe.html

We also update linux/mksysnum.go to ensure that the generated syscall constants
written to the zsysnum_linux_*.go files are always sorted by their syscall numbers
in ascending order.

Updates golang/go#61416

Change-Id: Iedb0a86adb65faac9061b9a5969ffa09eb5b303a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/510795
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

unix/zsysnum_linux_riscv64.go

@@ -251,6 +251,8 @@ const (
 	SYS_ACCEPT4                 = 242
 	SYS_RECVMMSG                = 243
 	SYS_ARCH_SPECIFIC_SYSCALL   = 244
+	SYS_RISCV_HWPROBE           = 258
+	SYS_RISCV_FLUSH_ICACHE      = 259
 	SYS_WAIT4                   = 260
 	SYS_PRLIMIT64               = 261
 	SYS_FANOTIFY_INIT           = 262