compiler, runtime: move constants into shared package

Use a single package for certain constants that must be the same between
the compiler and the runtime.

While just using the same values in both places works, this is much more
obvious and harder to mess up. It also avoids the need for comments
pointing to the other location the constant is defined. And having it in
code makes it possible for IDEs to analyze the source.

In the future, more such constants and maybe algorithms can be added.

loader/goroot.go

@@ -256,6 +256,7 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"runtime/":                    false,
 		"sync/":                       true,
 		"testing/":                    true,
+		"tinygo/":                     false,
 		"unique/":                     false,
 	}