Plugin not respecting arm64 architectures

[pariser]

Despite the fact that Graal has released builds for arm64 as of 21.0, it looks like this plugin is not respecting my architecture and always defaulting to amd64

gradle-graal/src/main/java/com/palantir/gradle/graal/DownloadGraalTask.java

Lines 132 to 139 in db34e6e

	private String getArchitecture() {
	switch (Platform.architecture()) {
	case AMD64:
	return "amd64";
	default:
	throw new IllegalStateException("No GraalVM support for " + Platform.architecture());
	}
	}

Note that the presence of the substring "64" does not differentiate between amd64 and aarch64 (arm64).

gradle-graal/src/main/java/com/palantir/gradle/graal/Platform.java

Lines 47 to 53 in db34e6e

	public static Architecture architecture() {
	String arch = System.getProperty("os.arch");
	if (arch.contains("64")) {
	return Architecture.AMD64;
	}
	return Architecture.UNKNOWN;
	}