runtime: mark riscv64 goexit with TOPFRAME

This corrects backtraces in gdb, making the runtime gdb tests pass on
linux/riscv64.

Change-Id: Icdb71450f2ab58debb1fe18455793133108971d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/231998
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/asm_riscv64.s

@@ -446,7 +446,7 @@ CALLFN(·call1073741824, 1073741824)
 // func goexit(neverCallThisFunction)
 // The top-most function running on a goroutine
 // returns to goexit+PCQuantum.
-TEXT runtime·goexit(SB),NOSPLIT|NOFRAME,$0-0
+TEXT runtime·goexit(SB),NOSPLIT|NOFRAME|TOPFRAME,$0-0
 	MOV	ZERO, ZERO	// NOP
 	JMP	runtime·goexit1(SB)	// does not return
 	// traceback from goexit1 must hit code range of goexit