cl,build: support //export with different names on embedded targets

Add support for using different C symbol names in //export directives
when compiling for embedded targets (enabled via -target flag).

This is for TinyGo compatibility on embedded platforms where hardware
interrupt handlers often require specific naming conventions.

Implementation:
- Add EnableExportRename() to control export rename behavior
- Add isExport parameter to initLink callback for context awareness
- Update initLinknameByDoc() to handle export rename logic:
  * When isExport && enableExportRename: allow different names
  * Otherwise: require name match
- Clean error handling in initLink():
  * export + enableExportRename: silent return (already processed)
  * export + !enableExportRename: panic with clear error message
  * other cases: warning for missing linkname

Example:
  //export LPSPI2_IRQHandler
  func interruptLPSPI2() { ... }

The Go function is named interruptLPSPI2 but exported as
LPSPI2_IRQHandler for the hardware vector table.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cl/compile.go

@@ -53,6 +53,11 @@ var (
 	enableDbg         bool
 	enableDbgSyms     bool
 	disableInline     bool
+
+	// enableExportRename enables //export to use different C symbol names than Go function names.
+	// This is for TinyGo compatibility when using -target flag for embedded targets.
+	// Currently, using -target implies TinyGo embedded target mode.
+	enableExportRename bool
 )
 
 // SetDebug sets debug flags.
@@ -73,6 +78,12 @@ func EnableTrace(b bool) {
 	enableCallTracing = b
 }
 
+// EnableExportRename enables or disables //export with different C symbol names.
+// This is enabled when using -target flag for TinyGo compatibility.
+func EnableExportRename(b bool) {
+	enableExportRename = b
+}
+
 // -----------------------------------------------------------------------------
 
 type instrOrValue interface {