Clean-up libc usage

core/c/libc/errno.odin

@@ -98,6 +98,14 @@ when ODIN_OS == .Haiku {
 	ERANGE :: B_POSIX_ERROR_BASE + 17
 }
 
+when ODIN_OS == .JS {
+	_ :: libc
+	_get_errno :: proc "c" () -> ^int {
+		@(static) errno: int
+		return &errno
+	}
+}
+
 // Odin has no way to make an identifier "errno" behave as a function call to
 // read the value, or to produce an lvalue such that you can assign a different
 // error value to errno. To work around this, just expose it as a function like