runtime: mark morestack_noctxt SPWRITE for linux/loong64

ref. CL 425396

Updates golang#54332.

Change-Id: I1a235b0cca4dbf79cf61cf5f40b594fc2d940857
Reviewed-on: https://go-review.googlesource.com/c/go/+/446895
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: WANG Xuerui <[email protected]>
Run-TryBot: Wayne Zuo <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: xiaodong liu <[email protected]>

Author: abner-chenc

src/runtime/asm_loong64.s

@@ -266,6 +266,13 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	UNDEF
 
 TEXT runtime·morestack_noctxt(SB),NOSPLIT|NOFRAME,$0-0
+	// Force SPWRITE. This function doesn't actually write SP,
+	// but it is called with a special calling convention where
+	// the caller doesn't save LR on stack but passes it as a
+	// register (R5), and the unwinder currently doesn't understand.
+	// Make it SPWRITE to stop unwinding. (See issue 54332)
+	MOVV    R3, R3
+
 	MOVV	R0, REGCTXT
 	JMP	runtime·morestack(SB)