feat(asm): emit linkable ELF and Mach-O objects with external symbols

Assisted-by: Qwen 3.8 Max Preview

asm/assemble.go

@@ -30,9 +30,12 @@ func Assemble(t *ast.Text) ([]byte, map[string]int, error) {
 // linkInfo carries file-level symbol context into a single-function assembly:
 // the set of static symbols a GLOBL in the same file defines.  A nil link
 // rejects SB operands outright (single-function assembly cannot resolve
-// them).
+// them).  When allowExternal is set, a reference to a symbol no GLOBL in the
+// file defines is recorded as an external relocation instead of failing —
+// the object-file emitters resolve it at link time.
 type linkInfo struct {
-	symbols map[string]bool
+	symbols       map[string]bool
+	allowExternal bool
 }
 
 // sbPatch is a function-relative static-symbol relocation: the disp32 field
@@ -425,7 +428,9 @@ func operandFromAST(op *ast.Operand, size int, fi frameInfo, link *linkInfo) (Op
 				if a.Sym.Static {
 					return nil, fmt.Errorf("undefined symbol %q", a.Sym.Name)
 				}
-				return nil, fmt.Errorf("external symbol %q needs object-file emission", a.Sym.Name)
+				if !link.allowExternal {
+					return nil, fmt.Errorf("external symbol %q needs object-file emission", a.Sym.Name)
+				}
 			}
 			return sbMem{size: size, name: a.Sym.Name, addend: a.Sym.Offset}, nil
 		}