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>

_demo/embedded/export_test/main.go

@@ -0,0 +1,22 @@
+package main
+
+//export LPSPI2_IRQHandler
+func interruptLPSPI2() {
+	println("LPSPI2 interrupt handled")
+}
+
+//export SysTick_Handler
+func systemTickHandler() {
+	println("System tick")
+}
+
+//export Add
+func Add(a, b int) int {
+	return a + b
+}
+
+func main() {
+	interruptLPSPI2()
+	systemTickHandler()
+	println("Add(2, 3) =", Add(2, 3))
+}