[path/filepath] Change join() to take a []string instead of varargs

This makes passing an allocator easier, as you no longer have to resort to
named arguments:
Before:
  `join(a, b, c)` became `join(elems={a, b, c}, allocator=ally)`
After:
  `join({a, b, c})` becomes `join({a, b, c}, ally)`

core/c/frontend/preprocessor/preprocess.odin

@@ -1276,7 +1276,7 @@ preprocess_internal :: proc(cpp: ^Preprocessor, tok: ^Token) -> ^Token {
 				if start.file != nil {
 					dir = filepath.dir(start.file.name)
 				}
-				path := filepath.join(dir, filename)
+				path := filepath.join({dir, filename})
 				if os.exists(path) {
 					tok = include_file(cpp, tok, path, start.next.next)
 					continue