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/mkpost.go

@@ -112,6 +112,12 @@ func main() {
 		}
 	}
 
+	if goos == "linux" && goarch != "riscv64" {
+		// The RISCV_HWPROBE_ constants are only defined on Linux for riscv64
+		hwprobeConstRexexp := regexp.MustCompile(`const\s+\(\s+RISCV_HWPROBE_[^\)]+\)`)
+		b = hwprobeConstRexexp.ReplaceAll(b, nil)
+	}
+
 	// Intentionally export __val fields in Fsid and Sigset_t
 	valRegex := regexp.MustCompile(`type (Fsid|Sigset_t) struct {(\s+)X__(bits|val)(\s+\S+\s+)}`)
 	b = valRegex.ReplaceAll(b, []byte("type $1 struct {${2}Val$4}"))
@@ -138,6 +144,11 @@ func main() {
 	ptraceRexexp := regexp.MustCompile(`type Ptrace((Psw|Fpregs|Per) struct {\s*})`)
 	b = ptraceRexexp.ReplaceAll(b, nil)
 
+	// If we have an empty RISCVHWProbePairs struct, we should delete it. Only riscv64 emits
+	// nonempty RISCVHWProbePairs structs.
+	hwprobeRexexp := regexp.MustCompile(`type RISCVHWProbePairs struct {\s*}`)
+	b = hwprobeRexexp.ReplaceAll(b, nil)
+
 	// Replace the control_regs union with a blank identifier for now.
 	controlRegsRegex := regexp.MustCompile(`(Control_regs)\s+\[0\]uint64`)
 	b = controlRegsRegex.ReplaceAll(b, []byte("_ [0]uint64"))