feat(asm): give riscv64 the END and GETCALLERPC the toolchain accepts

Assisted-by: GLM 5.3 Flash

asm/riscv_frame.go

@@ -34,6 +34,11 @@ import (
 type riscvFrameInfo struct {
 	autosize int // the real SP adjustment (locals + saved LR)
 
+	// leaf mirrors the toolchain's cursym.Leaf: the body makes no call, so
+	// the link register still holds the caller's address.  GETCALLERPC reads
+	// it there; a framed body reads the prologue's save at 0(SP) instead.
+	leaf bool
+
 	// Stack-split guard state: the toolchain emits the check for every
 	// non-NOSPLIT function whose autosize is nonzero (a zero autosize is
 	// "effectively NOSPLIT"); unlike amd64 and arm64 there is no leaf
@@ -44,12 +49,12 @@ type riscvFrameInfo struct {
 
 // riscvComputeFrame derives the frame layout for a TEXT function.
 func riscvComputeFrame(t *ast.Text) riscvFrameInfo {
-	frame := frameSize(t)
-	if frame != 0 || !riscvIsLeaf(t) {
+	leaf := riscvIsLeaf(t)
+	if frame := frameSize(t); frame != 0 || !leaf {
 		// FixedFrameSize = 8: space for the saved link register.  A
 		// zero-frame non-leaf function still opens an 8-byte frame for LR.
 		autosize := frame + 8
-		fi := riscvFrameInfo{autosize: autosize}
+		fi := riscvFrameInfo{autosize: autosize, leaf: leaf}
 		if !hasNoSplitFlag(t) {
 			fi.needSplit = true
 			switch {
@@ -63,7 +68,9 @@ func riscvComputeFrame(t *ast.Text) riscvFrameInfo {
 		}
 		return fi
 	}
-	return riscvFrameInfo{}
+	// A zero-frame leaf: no prologue, no guard, and the link register still
+	// holding the caller's address.
+	return riscvFrameInfo{leaf: leaf}
 }
 
 // hasNoSplitFlag reports whether the TEXT directive carries NOSPLIT.