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/test.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+set -e
+
+echo "Testing //export with different symbol names..."
+
+# Set LLGO_ROOT to repository root
+export LLGO_ROOT=/workspace
+
+# Build the test program
+echo "Building test program..."
+/workspace/llgo build -o test_export main.go || exit 1
+
+# Check for expected symbols using nm
+echo "Verifying symbols with nm..."
+
+# Should find LPSPI2_IRQHandler (not interruptLPSPI2)
+if nm test_export | grep -q "T LPSPI2_IRQHandler"; then
+    echo "✓ Symbol LPSPI2_IRQHandler found"
+else
+    echo "✗ Symbol LPSPI2_IRQHandler not found"
+    echo "Available symbols:"
+    nm test_export | grep " T "
+    exit 1
+fi
+
+# Should find SysTick_Handler (not systemTickHandler)
+if nm test_export | grep -q "T SysTick_Handler"; then
+    echo "✓ Symbol SysTick_Handler found"
+else
+    echo "✗ Symbol SysTick_Handler not found"
+    exit 1
+fi
+
+# Should find Add (same name)
+if nm test_export | grep -q "T Add"; then
+    echo "✓ Symbol Add found"
+else
+    echo "✗ Symbol Add not found"
+    exit 1
+fi
+
+echo ""
+echo "All symbol checks passed! ✓"