compiler: remove unnecessary main.main call workaround

Since https://github.com/tinygo-org/tinygo/pull/1571 (in particular, the first
commit that sets the main package path), the main package is always named
"main". This makes the callMain() workaround in the runtime unnecessary and
allows directly calling the main.main function with a //go:linkname pragma.

src/runtime/runtime.go

@@ -10,8 +10,7 @@ const Compiler = "tinygo"
 // package.
 func initAll()
 
-// callMain is a placeholder for the program main function.
-// All references to this are replaced with references to the program main function by the compiler.
+//go:linkname callMain main.main
 func callMain()
 
 func GOMAXPROCS(n int) int {