runtime: implement cputicks with the stable counter on loong64

The stable counter is described in Section 2.2.10.4, LoongArch Reference Manual Volume 1.

Ref: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: I160b695a8c0e38ef49b21fb8b41460fd23d9538c
Reviewed-on: https://go-review.googlesource.com/c/go/+/421656
Reviewed-by: Meidan Li <limeidan@loongson.cn>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/asm_loong64.s

@@ -86,6 +86,12 @@ TEXT runtime·mstart(SB),NOSPLIT|TOPFRAME,$0
         JAL     runtime·mstart0(SB)
         RET // not reached
 
+// func cputicks() int64
+TEXT runtime·cputicks(SB),NOSPLIT,$0-8
+	RDTIMED	R0, R4
+	MOVV	R4, ret+0(FP)
+	RET
+
 /*
  *  go-routine
  */

src/runtime/cputicks.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !arm && !arm64 && !loong64 && !mips64 && !mips64le && !mips && !mipsle && !wasm
+//go:build !arm && !arm64 && !mips64 && !mips64le && !mips && !mipsle && !wasm
 
 package runtime
 

src/runtime/os_linux_loong64.go

@@ -9,10 +9,3 @@ package runtime
 func archauxv(tag, val uintptr) {}
 
 func osArchInit() {}
-
-//go:nosplit
-func cputicks() int64 {
-	// Currently cputicks() is used in blocking profiler and to seed fastrand().
-	// nanotime() is a poor approximation of CPU ticks that is enough for the profiler.
-	return nanotime()
-}