Currently, there is no (non-legacy) way to specify the build platform when building the JDK. This can become problematic, especially when --openjdk-target is usually recommended for cross compiling, but cannot be used alongside --build if and when it is required (Building a Linux binary with WSL or correcting platform autodetection when it fails). This patch allows for explicitly specifying the build platform together with --openjdk-target and overriding the one obtained from config.guess, in lieu of these issues