fix(c/os): add missing clearenv for macOS

c/os/os.go

@@ -28,7 +28,8 @@ import (
 )
 
 const (
-	LLGoPackage = "decl"
+	LLGoFiles   = "_os/os.c"
+	LLGoPackage = "link"
 )
 
 const (
@@ -150,9 +151,6 @@ func Putenv(env *c.Char) c.Int
 //go:linkname Unsetenv C.unsetenv
 func Unsetenv(name *c.Char) c.Int
 
-//go:linkname Clearenv C.clearenv
-func Clearenv()
-
 // -----------------------------------------------------------------------------
 
 //go:linkname Fchdir C.fchdir