Move runtime os specific freestanding stuff to a separate file

core/runtime/os_specific.odin

@@ -1,26 +1,6 @@
+//+build !freestanding
 package runtime
 
-when ODIN_OS == "freestanding" {
-	_OS_Errno  :: distinct int;
-	_OS_Handle :: distinct uintptr;
-
-	os_stdout :: proc "contextless" () -> _OS_Handle {
-		return 1;
-	}
-	os_stderr :: proc "contextless" () -> _OS_Handle {
-		return 2;
-	}
-
-	// TODO(bill): reimplement `os.write`
-	os_write :: proc(fd: _OS_Handle, data: []byte) -> (int, _OS_Errno) {
-		return 0, -1;
-	}
-
-	current_thread_id :: proc "contextless" () -> int {
-		return 0;
-	}
-
-} else {
 import "core:os"
 
 _OS_Errno  :: distinct int;
@@ -42,4 +22,3 @@ when ODIN_OS == "freestanding" {
 current_thread_id :: proc "contextless" () -> int {
 	return os.current_thread_id();
 }
-}

core/runtime/os_specific_freestanding.odin

@@ -0,0 +1,21 @@
+//+build freestanding
+package runtime
+
+_OS_Errno  :: distinct int;
+_OS_Handle :: distinct uintptr;
+
+os_stdout :: proc "contextless" () -> _OS_Handle {
+	return 1;
+}
+os_stderr :: proc "contextless" () -> _OS_Handle {
+	return 2;
+}
+
+// TODO(bill): reimplement `os.write`
+os_write :: proc(fd: _OS_Handle, data: []byte) -> (int, _OS_Errno) {
+	return 0, -1;
+}
+
+current_thread_id :: proc "contextless" () -> int {
+	return 0;
+}