feat(riscv): add prologue, epilogue and frame pseudo-register support

Assisted-by: Kimi K3

cmd/gasm/main.go

@@ -374,8 +374,9 @@ requires -p, the package path, and the installed Go toolchain).
 		return 2
 	}
 	path := fs.Arg(0)
-	if arch.FromFilename(path) != arch.AMD64 {
-		fmt.Fprintln(os.Stderr, "gasm asm: only amd64 is supported in this Phase 2 increment")
+	targetArch := arch.FromFilename(path)
+	if targetArch != arch.AMD64 && targetArch != arch.RISCV {
+		fmt.Fprintln(os.Stderr, "gasm asm: only amd64 and riscv64 are supported")
 		return 1
 	}
 	src, err := readSource(path)
@@ -391,7 +392,12 @@ requires -p, the package path, and the installed Go toolchain).
 		return 1
 	}
 
-	img, err := asm.AssembleFile(f)
+	var img *asm.Image
+	if targetArch == arch.RISCV {
+		img, err = asm.AssembleFileRISCV(f)
+	} else {
+		img, err = asm.AssembleFile(f)
+	}
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s: %v\n", path, err)
 		return 1