feat: support //export with different symbol names for embedded targets

Implements support for TinyGo-style //export directive that allows
exporting functions with symbol names different from the Go function name.

This is essential for embedded development where hardware specifications
require specific symbol names (e.g., ARM Cortex-M interrupt handlers like
LPSPI2_IRQHandler, SysTick_Handler).

Changes:
- Modified cl/import.go to support two export formats:
  1. //export ExportName (TinyGo-style, export name can differ from function name)
  2. //export FuncName ExportName (Go-style, explicit function and export names)
- Added test case in _demo/embedded/export_test/ with nm verification

Example usage:
  //export LPSPI2_IRQHandler
  func interruptLPSPI2() {
      // exported as LPSPI2_IRQHandler
  }

Fixes #1378

🤖 Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

cl/import.go

@@ -278,7 +278,8 @@ func (p *context) initLinknameByDoc(doc *ast.CommentGroup, fullName, inPkgName s
 		for n := len(doc.List) - 1; n >= 0; n-- {
 			line := doc.List[n].Text
 			ret := p.initLinkname(line, func(name string) (_ string, _, ok bool) {
-				return fullName, isVar, name == inPkgName
+				// support empty name for //export directive
+				return fullName, isVar, name == inPkgName || name == ""
 			})
 			if ret != unknownDirective {
 				return ret == hasLinkname
@@ -312,10 +313,26 @@ func (p *context) initLinkname(line string, f func(inPkgName string) (fullName s
 		p.initLink(line, len(llgolink), false, f)
 		return hasLinkname
 	} else if strings.HasPrefix(line, export) {
-		// rewrite //export FuncName to //export FuncName FuncName
-		funcName := strings.TrimSpace(line[len(export):])
-		line = line + " " + funcName
-		p.initLink(line, len(export), true, f)
+		// support //export ExportName
+		// format: //export ExportName or //export FuncName ExportName
+		exportName := strings.TrimSpace(line[len(export):])
+		var inPkgName string
+		if idx := strings.IndexByte(exportName, ' '); idx > 0 {
+			// format: //export FuncName ExportName (go-style)
+			inPkgName = exportName[:idx]
+			exportName = strings.TrimLeft(exportName[idx+1:], " ")
+		} else {
+			// format: //export ExportName (tinygo-style)
+			// use empty string to match any function
+			inPkgName = ""
+		}
+		if fullName, _, ok := f(inPkgName); ok {
+			p.prog.SetLinkname(fullName, exportName)
+			p.pkg.SetExport(fullName, exportName)
+		} else {
+			fmt.Fprintln(os.Stderr, "==>", line)
+			fmt.Fprintf(os.Stderr, "llgo: export %s not found and ignored\n", inPkgName)
+		}
 		return hasLinkname
 	} else if strings.HasPrefix(line, directive) {
 		// skip unknown annotation but continue to parse the next annotation