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>

internal/build/build.go

@@ -214,6 +214,11 @@ func Do(args []string, conf *Config) ([]Package, error) {
 		conf.Goarch = export.GOARCH
 	}
 
+	// Enable different export names for TinyGo compatibility when using -target
+	if conf.Target != "" {
+		cl.EnableExportRename(true)
+	}
+
 	verbose := conf.Verbose
 	patterns := args
 	tags := "llgo,math_big_pure_go"